The New York Times, by Staff

By Robert D. McFadden

An explosion apparently caused by a car bomb in an underground garage shook the World Trade Center in lower Manhattan with the force of a small earthquake shortly after noon yesterday, collapsing walls and floors, igniting fires and plunging the city's largest building complex into a maelstrom of smoke, darkness and fearful chaos.

The police said the blast killed at least five people and left more than 650 others injured, mostly with smoke inhalation or minor burns, but dozens with cuts, bruises, broken bones or serious burns. The police said 476 were treated at hospitals and the rest by rescue and medical crews at the scene.

The explosion also trapped hundreds of people in debris or in smoke-filled stairwells and elevators of the towers overhead and forced the evacuation of more than 50,000 workers from a trade center bereft of power for lights and elevators for seven hours. No Bomb Fragments Found

The blast, which was felt throughout the Wall Street area and a mile away on Ellis and Liberty Islands in New York Harbor, also knocked out the police command and operations centers for the towers, which officials said rendered the office complex's evacuation plans useless. [ Page 23. ]

James Fox, an assistant director of the Federal Bureau of Investigation in charge of the agency's New York office, said that no bomb fragments were found but that a joint terrorist task force of Federal agents and city detectives had examined the wreckage and believed that a car bomb had caused the explosion.

There was no warning of an impending explosion, Police Commissioner Raymond W. Kelly said. Jack Killorin, a spokesman in Washington for the Treasury Department's Bureau of Alcohol, Tobacco and Firearms, said that after the blast, authorities received at least nine telephone calls claiming responsibility.

Mr. Killorin said the first call was made 15 minutes after the blast to a non-emergency number of a New York Police Department precinct by an individual who mentioned the conflict in Bosnia. He said other claims were made between an hour and several hours after the event by callers who cited that and a variety of other reasons for the attack. He declined to elaborate.

Some law-enforcement officials said an explosion of such size, without a claim of responsibility in advance, might suggest that it went off accidentally.

Mr. Kelly was more oblique about the cause of the blast, saying only that a car bomb or other type of explosive device was not being ruled out.

Four hours after the explosion, a bomb threat forced the evacuation of the Empire State Building in midtown Manhattan, and there were numerous other bomb threats in the city, the police said. But it was unclear if any were related to the World Trade Center explosion or only the macabre work of pranksters.

As the day ended, a series of investigations began -- into the cause of the explosion and its possible perpetrators, and into what went wrong in what many called a botched evacuation, with no alarms and no instructions for thousands caught in dark, smoky stairwells, in stark contrast to carefully laid plans.

Mayor David N. Dinkins, visiting in Osaka, Japan, was notified by City Hall and, in a telephone news conference, called the Fire Department response the largest for any non-natural disaster in the city's history. He said he had spoken with President Clinton and had thanked him for the cooperation of Federal investigators.

The effects of the blast radiated outward, disrupting most non-cable television transmissions throughout the metropolitan area, halting traffic in most of lower Manhattan and PATH train service under the Hudson River from the trade center to New Jersey, and transforming an ordinary Friday in the financial district into an afternoon of turmoil, death and destruction.

On a day of high drama, tragedy and heroism, there were a thousand stories: rescuers digging frantically for victims in the collapsed PATH station under the towers, soot-streaked evacuees groping for hours in the city's tallest buildings, a woman in a wheelchair carried down 66 stories by two friends, a pregnant woman airlifted by helicopter from a tower roof, and the tales of many others stumbling out, gasping for air, terrified but glad to be alive.

And among the most poignant was that of a class of kindergartners from Public School 95 in Gravesend, Brooklyn. Caught on the 107th floor observatory, they took all day to walk down, singing to keep up their spirits.

Many of those who walked down scores of flights from the upper reaches of the trade center towers said there had been no alarm bells and no instructions from building personnel or emergency workers. While little panic was reported, witnesses said confusion reigned in the darkness of crowded stairwells where smoke billowed and unknown dangers lurked below.

Many put moist towels or handkerchiefs to their faces against the smoke. Others, frightened, remained in their offices, hoping for rescue. As smoke seeped in under the doors, some broke windows to get air. Dozens of people, meantime, were trapped for hours in elevators frozen between floors, among them another class of kindergartners from P.S. 95.

The worst fires were extinguished by midafternoon. By then extensive efforts to assist those caught on the upper floors were already well underway. But the trade center, with 250 elevators and miles of corridors and stairways, posed a major challenge and long after dark last night rescue workers continued to search the labyrinth for stragglers and others still trying to get out.

On a day of confusion, the police and the Emergency Medical Service repeatedly revised the number of people killed by the blast. By early evening the police said five had been killed while the medical service said seven were dead. Several hours later the police increased the number to seven, but shortly after 11 P.M., the police scaled back the figure to five, saying they could not confirm the medical service's report of seven fatalities. There was no clear explanation for the discrepancies.

The blast, which erupted at 12:18 P.M. on the second level of a four-story underground parking garage beneath the trade center's 110-story twin towers and the complex's Vista Hotel, sent cars hurtling like toys, blew out a 100-foot wall and sent the floor collapsing down several stories, creating a crater 60 feet wide that reached deep into the bowels of the parking complex. 'Everything Was on Fire'

It also collapsed the ceiling of a mezzanine in the adjacent Port Authority Trans-Hudson train station, leaving dozens trapped under rubble on a concourse one floor above the platforms where hundreds awaited trains. Witnesses and rescue workers told of a blast of incredible force -- of bodies hurtling through the air, of cars wrapped around pillars, of people burning and scores trapped.

"We crawled under pipes when we arrived and everything was on fire," said Edward Bergen, a 38-year-old firefighter who was one of the first to reach the scene of the blast. "Suddenly, a guy came walking out of the flames, like one of those zombies in the movie, 'The Night of the Living Dead.' His flesh was hanging off. He was a middle-aged man."

Fire Capt. Timothy Dowling, of Engine Company 6, recalled a ghastly scene of fires lapping in the darkness, illuminating a smoking hell of twisted cars and broken concrete. "It looked like a bomb had exploded because of the amount of fire and damage to the floors. All we could do was put water on the flames." 'The Dust Was Blinding'

Ken Olson, 34, a pipefitter, was in the basement when the explosion hit. "All of a sudden all hell broke loose," he said. "All the pipes ruptured, the dust was blinding. Luckily we all stayed together and got out."

Nearby, Vito de Leo, 32, an air-conditioning mechanic, was eating lunch at his desk with other basement trade center workers. Suddenly, the desk rose up, came down and landed on top of him. But its well protected him from a rain of falling debris. "The furniture collapsed, the walls collapsed, the ceiling collapsed," he said. "There was total blackness. I thought I was dead."

Later, wading through knee-deep water amid gas pipelines danging overhead in the garage, a cadre of firefighters, police officers and other rescue workers found two bodies in a lunchroom used by mechanics, another body in the mangled wreckage of a car, and more victims under the debris in the garage.

The five victims -- three men, one woman and one unidentified -- were all believed to heve been killed by the blast. They were not immediately identified but the Port Authority said that they were all believed to be authority workers or people working under contract to the agency. The authorities said that more bodies might be found in the rubble as the search went on.

The police said 420 workers and visitors at the trade center were treated at hospitals, along with 44 firefighters, 11 police officers and one Emergency Medical Service worker.

Meantime, as several fires erupted around the scene of the explosion, heavy smoke billowed up through the corridors, elevator shafts and stairwells of the trade center. Because of the time, shortly after noon, many workers were at lunch at nearby restaurants or at fast-food outlets on the ground floor, from where they easily escaped.

But the police estimated that as many as 50,000 people -- workers, tourists and other visitors were still in the building, many of them trapped on the highest floors -- and it was not merely the blast that shook the entire complex, not merely the growing volumes of smoke pouring upward, that frightened them.

Darkness and the unknown perils that awaited them below added to the fears. Much of the power to the trade center had been knocked out by the blast -- Consolidated Edison said four of its eight feeder cables to the center were shut down.

And within an hour, at the request of the Fire Department, which was trying to protect rescue workers and firefighters in dangerous areas, all power to the trade center was shut off by Con Edison, as well as natural gas and steam to the complex, which houses thousands of offices in six buildings bounded by Church, West, Liberty and Vesey Streets.

Pat Richardi, a Con Edison spokeswoman, said that no hazardous materials, such as polychlorinated biphenals, were in any of the transformers or cables of the trade center, which was completed 22 years ago.

Many of the people climbing down stairs told of having to stop frequently because of panic below; some let pregnant women and old people go through; some nearly passed out with exhaustion; others told of tense minutes in which they sat down on the steps, trying to regain breath in stifling, smoky air.

"It was like sardines, cattle, a herd," said Larry Bianculli, 31, of Hicksville, L.I., who walked down 104 floors with his wool scarf over his sooty face.

Sherri Chambers, 21, a bank employee, said it took her two and a half hours to descend from the 60th floor. "You couldn't even see it was so smoky," she said. "I kept wanting to sit down, but I didn't because if I sat down I thought I wouldn't get up. Firefighter Bill Chupa, 40, of Ladder 20, said many people were trapped in elevators and screaming for help. He said firefighters broke open elevator doors and found people in groups of 8 or 10, lying in darkness on the floor to escape the smoke.

After freeing those in elevators, the rescuers turned to the stairwells and began escorting people down. By midafternoon, there was a steady stream of survivors coming from the towers, many with faces blackened by smoke and gasping for air.

Some of the most spectacular evacuations came when police helicopters landed on the roofs of the trade center towers and carried away 23 people, including a pregnant woman.

Don Burke, who works for the Port Authority on the 66th floor, ran back to his office when he discovered there was a fire and, with a colleague, carried Cathy Collins, a lawyer who uses a wheelchair, to safety in relays.

In a shopping area on the ground floor of the trade center's World Financial Center at 250 Vesey Street, medical and rescue workers set up a triage area of folding chairs, oxygen tanks, blood pressure devices, blankets and other medical aids.

While there was little panic, aides to Gov. Mario M. Cuomo told of a pregnant woman screaming as they descended from the Governor's 57th floor offices in a chaos of darkness and disorder.

The Governor, who was in Albany, said President Clinton had called him to express concern and offer aid from the Federal Emergency Management Agency. Mr. Cuomo also raised some questions: "What emergency devices were available? Did they work? Why were there no lights? Why were there no public announcements. The Port Authority will be called upon to answer, and I'm sure they will."

Power was partly restored to the trade center towers at 7:20 P.M., and by 9:30 rescue workers said everyone had been evacuated.

The ceiling collapse in the PATH station forced a halt to all train service to New Jersey from the trade center, but PATH service from midtown, operating on another line, continued to operate through the day. Subway trains were rerouted and continued to run, but streets throughout the area were closed to clear the way for emergency vehicles.

These New York Times reporters and photographers contributed to the coverage of the explosion at the World Trade Center: Ralph Blumenthal, Fred R. Conrad, Celia W. Dugger, Seth Faison, Ian Fisher, Lindsey Gruson, Dennis Hevesi, Lynette Holloway, Marvine Howe, Edward Keating, Clifford J. Levy, James C. McKinley Jr., Steven Lee Myers, Alison Mitchell, Maria Newman, Larry Olmstead, Garry Pierre-Pierre, Todd S. Purdum, Dith Pran, Selwyn Raab, Lynda Richardson, Calvin Sims, Ronald Sullivan, Ruby Washington, Craig Wolff.

By N.R. Kleinfeld

It depended on where you were in the towers when it came. For some the warning was a trembling underfoot or just a blank computer screen and flickering lights. For others, it was a shocking noise. One woman was blown out of her high heels. Another, desk chair and all, sank into the floor. And then, instantly it seemed, came the billowing smoke and the chilling realization that you had to get out of there. The Face of Death

There were those who panicked, those who coolly absorbed it, those who got sick to their stomach and those who saw the face of death. No one was sure what had happened; did a plane hit the building, was it an earthquake, had lightning struck? Many wondered why there seemed to be no evacuation plan and no guidance -- not realizing that the blast had knocked out the center's operations center.

But thousands of people in the World Trade Center yesterday afternoon knew they were in the grip of one of the most dreaded urban nightmares: they were in the city's tallest building and something was very wrong.

Joann Hilton was low. And, in this disaster, that was the worst place to be. A secretary working for the Port Authority of New York and New Jersey, she was at her desk in the command office on the B1 level of the building, musing about the weekend.

"All of a sudden, we heard this big boom," she said. "It sounded like an earthquake. And then the floor just collapsed and me and my chair sank into the floor. The ceiling started to come down, too, and I'm in my chair in the floor. Some of the lights went out. And then it was all dark. Like a cave. Somebody pulled me out of that floor and we beat it out of there."

Denise Bosco was high. She was on the 82d floor, where she too works as a secretary for the Port Authority. "The whole building shook," she said. "The lights flashed on and off. The computers went down. Then, instantly, there was smoke. I was terrified. People panicked. They started pushing and shouting to get out. Some of them were throwing up. I said, 'Oh dear God, what is it? What is it? Is it my time? Is this the way?' " Wrapped in a Bath Towel

Her coat was still in her office and she was wrapped in a white bath towel as she stood outside. She broke down into tears. "It was horrible," she said. "There was this awful feeling that we might not be able to get out. We were in the mighty, tall tower but we weren't getting out."

They didn't know whether to stay put or flee, but instinct said to run. Along with her co-workers, she sped for the stairwells and began the seemingly endless descent. Smoke was thick in the stairs, and it was dark. And congested. People fumbled their way down. A few were lucky enough to have found flashlights in their offices. Others lighted matches. As they reached another floor, they would have to stop, because new escapees were joining the mass exodus.

Courtesy held up. When people saw a pregnant woman, they would call out, "Pregnant woman on the right," and usher her by. Two pregnant woman inched their way down together, crying the whole time. Others summoned the strength to carry the more feeble on their backs.

"I kept going and going," Ms. Bosco said, "never sure how many more flights I would make. It was unbelievable."

It took her an hour and a half before she was out in the daylight, shivering in the cold. 'I'm Not Going Back'

She has already resolved her future association with the trade center, where 50,000 people are employed and 80,000 visit daily. "I'll never go into that building again," she said. "I'm sorry. I'm not going back in there ever."

Phenomenal luck played its hand for some. Brenda Russillo, a Secret Service agent, was returning with two other agents to their office. They swung their car into the parking garage beneath 1 World Trade Center. As she stepped from the car, Ms. Russillo was blown out of her high heels five feet into the air. Landing hard on her chest, she instinctively yanked her coat over her head.

"I screamed, 'We've got to get out of here!' " she said. "I couldn't see. I couldn't catch my breath."

One of the other agents was propelled 20 feet and cut his eye. "I said, 'It's a bomb, it's a bomb!' " he said. " 'There's got to be another one!' "

The third agent thought someone had knocked him over. "I thought somebody is kidding around and slapped me as hard as he could," he said.

Then guilt overtook him. He had been driving and thought he must have been responsible for the explosion. "I thought I'd backed into an air compressor," he said. "I thought I caused the whole thing."

Not far from where they lay, a gaping hole had opened up and two floors had collapsed into rubble. Behind them, the metal doors of the garage were twisted. A "Sorry Garage Full" sign lay bent and on its side.

All three agents escaped serious injury. Her heels gone, Ms. Russillo left a hospital later wearing green hospital slippers.

In a ballroom of the Vista Hotel, right above what appeared to be the center of the eruption, tables and dressing rooms had been prepared for a children's beauty contest yesterday. There were cracks in the floor. A big piece of the ceiling dangled crazily. Rubble was everywhere. But the children were not yet there.

Courage also made its appearance. As soon as he realized there was fire, Don Burke, a Port Authority employee on the 66th floor, scrambled back into his office and found Kathleen Collins, a lawyer who uses a wheelchair. Mr. Burke and a colleague assisted her down the 66 flights. She would bump herself down, like a child playing, for a few steps, then the men would take turns carrying her. Near Death in an Elevator

Michael Dugan, a firefighter, tackled the elevators in 1 World Trade Center. "We opened an elevator door and found people who had been in there for at least two hours," he said. "There were 10 people lying on the floor. None of them was moving. It was like a scene from a movie. All we could hear was one woman crying. The rest were semiconscious with coats thrown over their faces. They were five or 10 minutes from death."

Some people sought safety higher up. A pregnant woman was plucked off the roof of 2 World Trade Center by a police helicopter. Later, a helicopter also picked up a police officer after he was overcome by smoke.

Some people stayed put. David McDonnell, an engineer, remained in his 51st-floor office, knocking out some windows and waiting until rescuers arrived. Destruction and Confusion

Confusion was rampant. Those who fled said they had no formal warnings, no sirens or alarms. Most said they didn't encounter rescue workers until they had reached the last couple of dozen flights. The director of the World Trade Center, Charles Maikish, said later that damage from the blast had "destroyed" the center's elaborate evacuation plan.

Many complained afterward about the lack of guidance. A 27-year-old trader working at the commodities exchange on the eighth floor of 4 World Trade Center said he was enraged that 4,000 people on the trading floor were not evacuated until 1:30, more than an hour after the explosion.

Frances Morrill, a travel agency manager who escaped from the 25th floor, said, "No one was telling us whether we were walking away from the danger or whether we were all fleeing right down into the fire."

Some of the firefighters and police officers succumbed to smoke inhalation. One firefighter, Tom Shea, suffered a broken left kneecap trying to pull a man out of the basement. The floor collapsed under him and he fell two stories. No More Than a Rumbling

People chose their course of action in odd ways. Raquel Vidal, a legal assistant for Federal Home Loan Bank, was at her office on the 103d floor. That high up, she said, she detected no more than a rumbling. "We said, what was that?" she recalled. "My boss said, 'Let's wait a while before we do anything.' "

Ms. Vidal picked up the phone and called her sister and asked her to turn on the radio and see if there was any news about the World Trade Center. "She heard plenty," Ms. Vidal said, "and she shouted at me, there's been an explosion, get out of there now."

As she threaded her way down the stairs, she began to feel much older than her 43 years. "About the 40th floor, my knees starting to give in," she said. "I didn't think I was going to make it. But my co-workers kept egging me on. Let's keep going, they'd say. We only have 40 floors to go. We only have 30. We only have 20. So I kept going, and I'm not sure my knees will ever forgive me."

By Deborah Sontag

Traffic snarled to a standstill in lower Manhattan. Major arteries downtown closed, and a half-dozen subway lines were rerouted. Nervous relatives jammed the 911 emergency lines. And an anxiety that began in lower Manhattan at lunchtime yesterday grew infectious as the day grew long, generating a restless buzz throughout New York City.

As news spread that the explosion at the World Trade Center -- which the F.B.I. said may have been caused by a car bomb -- had been followed by a bomb scare at and evacuation of the Empire State Building, nerves began to fray. Drivers sitting bumper to bumper on Canal Street leaned on their horns, waiting over an hour to enter the Holland Tunnel. Subway riders jostled their way onto overcrowded trains, pondering their commuters' headaches as well as what seemed to be a different kind of urban terrorism. An Eerie Quiet on Closed Streets

"It's frightening," said Charles Sampey, 34 years old, a building superintendent riding the IRT No. 3 downtown at 6 P.M. "You'd expect more to get robbed in New York than to get hit by a terrorist bomb."

After an underground explosion rocked the World Trade Center at noontime, the Franklin D. Roosevelt Drive, the West Side Highway, the Brooklyn Battery Tunnel and most side streets in lower Manhattan were closed to all but emergency vehicles. It grew eerily quiet -- "like Sunday at dawn," said a Triborough Bridge and Tunnel Authority spokesman, Frank Pascual -- but for the cry of sirens. With their antennae atop the Trade Center, all except one New York television station -- WCBS, Channel 2 -- went off the air.

As trading on five commodities exchanges was disrupted and thousands of employees were evacuated from the smoke-filled office complex, transportation officials immediately began making emergency plans for an imminent rush hour. They added buses, trains and ferries to New Jersey to make up for the closing of the PATH Station at the Trade Center.

At 2 P.M., Trade Center office workers wearing pearl earrings and raccoon-like circles of soot around their nostrils began to flee lower Manhattan. They were a surreal presence, kicking off an early rush hour and an evening marked by a sense of unease.

A panicky uncertainty prompted 16,000 calls to 911 in the eight hours following the explosion as people grew concerned about relatives or just curious for more information. An average day would produce about 5,500 calls in the same period of time. In a 20,000 square-foot room on the ninth floor of One Police Plaza, 94 operators worked under flourescent lights in an atmosphere of controlled commotion. Another phone center, a hot line set up in the afternoon, drew about 3,000 calls.

Throughout the city, rumors abounded, such as the one that a Presidential limousine had been in the garage when a car bomb apparently exploded. But "the President's vehicle was not there," said Dale Wilson, a spokesman for the Secret Service in Washington.

Shortly after 4 P.M., firefighters and police evacuated all visitors and workers from the Empire State Building because of a bomb threat, which turned out to be a false alarm. More rumors, this time of a link with the World Trade Center explosion, rippled through the crowd.

"People were saying, 'Is it Bosnia? Somalia? Saddam Hussein?' " Julia Mantiales, a tourist from California, said last night. "Then it was, like, 'What's next? The Chrysler Building?' "

What was next was the Port Authority Terminal, which received a "general bomb scare" at 5:45 P.M., said Edward C. Forker, a Port Authority police inspector. While extra officers scoured the bustling terminal for unattended baggage, no bomb was found.

On a typical day, New York City receives 7 to 10 bomb threats. Yesterday, there were 69 threats between 2 and 9 P.M.

The Port Authority placed the region's three airports -- La Guardia, Kennedy International and Newark International -- at their highest level of security.

At the bus terminal, authorities expected about 218,000 commuters last evening, about 18 percent more than usual on 167 extra buses. The ferry service between Battery Park and Hoboken was transporting 6,000 commuters an hour on five boats, about 2,000 more people than it usually moves in an evening rush-hour.

Some New Yorkers, being New Yorkers, were remarkably blase. Winston Spencer, a delivery man and keyboard player who lives in Harlem, said, "It's New York's personality, the madness, the craziness." And Raymond Kowalski, a specialist at the New York Stock Exchange, said, "It's just another day at the park."

But Joan Weiss, pausing on a corner in TriBeCa, expressed the common feeling of vulnerability that many edgy New Yorkers voiced last night. "No one feels safe anymore," she said. "You never know if you're going to come home alive."

By Martin Gottlieb

Evacuation plans for the World Trade Center were rendered useless yesterday because the explosion devastated its police command and operations centers, Port Authority officials said.

As a result, tens of thousands of office workers were left to fend for themselves in terrifying confusion as they groped their way with no organized leadership down as many as 100 flights of stairs in darkness and choking smoke.

Charles Maikish, the director of the World Trade Center, said that it had an elaborate evacuation plan but that it was "destroyed" by the blast, which almost immediately knocked out most of the main power system, which serves both towers. Not only did the explosion severely damage the police desk and the operations center for the entire complex, but it knocked out their electricity, telephones, closed-circuit television monitors and public-address system.

Mr. Maikish said that a set of generators that could have powered the emergency systems was also lost when the blast severed lines that carry water to cool the generators. Only the Vista Hotel and 7 World Trade Center, across Vesey Street from the main complex, still had power.

Mark Marchese, a spokesman for the Port Authority, said later that the enmergency generators could also have powered a system that would have reversed the towers' exhaust fans to suck out the smoke.

Sal Samperi, the deputy director of the Port Authority police, said, "Our initial control desk was an officer with a walkie-talkie." Bomb-Proof, Studies Said

As described by Mr. Maikish, the public-address system and other components of the communications system are the linchpins of an evacuation plan that he said was practiced two or three times a year. It involves directing a network of fire safety marshals on every floor of each tower to the safest stairways, based on information received in large part through the television monitors.

Stephen R. Berger, the former director of the Port Authority, said that the bistate agency, which runs the buildings, had commissioned studies on terrorist attacks before the centennial of the Statue of Liberty in 1986; he said the studies concluded that the trade center could withstand a car bombing in an underground garage.

"They said you could sustain a car bomb," he said. "What they didn't tell us was you couldn't sustain it if it was perfectly placed."

Mr. Samperi said it was unclear at this point how well many of the fire marshals performed. Furious at Lack of Help

But as an army of city police officers, firefighters and ambulance teams massed in a 10-block area around the towers, it was clear that many trade center workers, their faces streaked with soot as they streamed from the buildings only to slip on snow-slicked pavements, were furious at being left to guess their way out. The rage swept from workers in Gov. Mario M. Cuomo's office on the 57th floor to those who happened to be in the building for lunch and credited their fast exit to the canniness of busboys.

"There was no information anywhere about anything," said Karen Eggleston, who was attending a training seminar on the 87th floor when the lights went out. "It was totally uncoordinated. You were on your own."

Once on the street, the evacuees, often shaken and gasping, received help from firefighters, ambulance workers and the Red Cross.

Those who weren't whisked to area hospitals huddled, blankets draped over their shoulders, in twos and threes. Many sobbed as they waited for blankets. At one point, dozens of people clamored around an ambulance, gasping, coughing and spitting as they grabbed for oxygen masks. Many, with red eyes and soot-blackened faces, simply sat on the icy ground, breathing heavily, as the snow fell.

Six people, including a pregnant woman, were taken from the roof of Tower 1 by a police helicopter. Emergency Officials Tried

As the day wore on, as many as 300 Port Authority police officers filtered into the building, aided by hundreds of city police, fire and Emergency Medical Service workers, to guide people through the choking smoke.

Rescue workers helped victims out of the building and over to an ambulance and a nearby Emergency Medical Service bus.

The emergency workers, from a host of agencies, also leaned against ambulances on occasion, gasping with exhaustion and from the inhalation of smoke during their forays into the center. Then they headed back into the complex to find more who were stranded.

Charles Knox, the Port Authority's director of public safety, said: "We do have evacuation plans for normal situations, but we have never experienced anything like this. This is the most devastating situation the World Trade Center has had to confront."

By Allison Mitchell

The school bus full of kindergartners pulled up outside Public School 95 in the Gravesend section of Brooklyn just before 8 P.M. Teachers and school officials formed a double line at its door. And then one by one, 5-years-olds in colorful parkas were handed off the bus, some in tears, some unfazed and some bewildered after a day of being trapped in the World Trade Center.

The day's vigil had come to an end. "Thank God. I never thought I'd be so happy to see my school," said Annamarie Tesoriero, who had led her 17 youngsters in songs and prayers to keep their spirits up through five hours stuck in a dark elevator, with no hint of when they would be rescued. "We expected the children to behave. We're old-fashioned that way."

Millie Rodriguez, a parent who had been helping on the field trip, said: "The teacher was great. She took out her glow-in-the-dark rosary and we prayed."

The worst part of the long hours in the dark elevator was the uncertainty, with no hint of what had happened, no calls or contact from the outside world, Mrs. Tesoriero said.

For Rosemarie Russo's class, the problem was the cold. When the building shook and smoke billowed into the 107th floor cafeteria, she had to lead her students outside to the observation deck to huddle in the frigid temperatures for three hours. They then staggered down 107 flights, taking several hours.

The ordeal over, parents and friends raced to grab children in the elementary school gym. "I love you, I love you," Steve Caridad crooned, lifting Sabrina Ortiz, the 5-year-old daughter of a close friend.

Lyndon Johnson promised his stepdaughter, Claudia Smith, anything she wanted, even a trip to Disney World. The joy was a babble of English, Italian and Chinese. "No more trips," Arthur Bradley, 26, lectured his mother, Rose, who had been one of the teachers on the field trip.

Ms. Russo burst into tears when she was applauded by the waiting parents. As her charges left, she bent and kissed several of them goodbye.

Most of the 5-year-olds didn't say a word, simply sucking on ice-cream pops handed to them for solace and clutching their parents' legs.

It was nearly 12 hours earlier when five kindergarten classes had set off in buses for the annual and usually uneventful trip to the World Trade Center. The morning passed as usual, and by about 12:30 three of the classes were already preparing to leave.

Lisa Perri had just herded her students onto one of the waiting buses when the building behind her exploded. "The bus rocked," she said. "One of the parents said, 'What is that?' I said it was a bomb."

Behind her, two more classes were stuck in the lobby, the children caught in the stampede of adults fleeing the building. "They were trapping the kids," said Miss Perri, who describing how the teachers pulled the children out.

Miss Perri called the school, which became an instant magnet for Brooklyn parents who had heard of the explosion on television and radio. Slowly, each class called in except one. For long, tense hours no one knew what had happened to Mrs. Tesoriero's missing class until the elevator moved and she managed to telephone from the 35th floor about 6 P.M., to the school where parents and teachers were maintaining an anxious vigil. 'I Started Crying'

The first three classes returned to the school in midafternoon. "I was fine until I saw all the kids' mothers," said Miss Perri, 25. "They were crying. I started crying. It was terrible."

Danielle Cancelleri had come to school in search of her 5-year-old son, Danny, who was in Miss Perri's class. But hours after he had returned safely and been sent home to the care of his father and relatives, Mrs. Cancelleri was still at the school to lend her support, unwilling to leave the other parents. "I know how I would have felt if my son was still there," she said. Many of the teachers also lingered long after working hours. "We'll stay until they all return," said Denise Cascini, a foruth-grade teacher waiting in the school lobby.

After all the children were safely back at the school, it was clear that one ritual was likely to be abandoned -- the annual World Trade Center trip. Said Ms. Russo, who had made the trip annually for about 10 years, "I will never go there again."

By Douglas Jehl

WASHINGTON, Feb. 26— The phenomenon is well known in Belfast, Lima, Bogota and Beirut, but if the explosion that rocked the World Trade Center today was indeed caused by a car bombing, as the F.B.I. believes, it would be the largest such attack in American history, experts said. 

Not since 1975, when 11 people were killed at La Guardia Airport, has anyone used a bomb, delivered in any fashion, to kill so many civilians in the United States. It is unknown whether a terrorist group was responsible for the explosion today.

"This is the largest-scale bombing on U.S. soil in modern history" in terms of fatalities and material damage, said Brian Jenkins, a security consultant with Kroll & Associates who is one of the nation's leading private experts on terrorism.

If the American setting is novel, however, the apparent use of a vehicle as a platform for the bomb represents only the latest use of what for more than four decades has been a favorite tool for terrorists. How to Pack a Big Punch

Ever since the Irish Republican Army first used the car bomb in 1971, Mr. Jenkins and other experts said, terrorist groups have found no better way to bring so much explosive power so close to a target.

In the last several weeks alone, car bombs have exploded repeatedly on the streets of cities in Colombia and on the streets of London. If the tactic until now has been barely known in the United States, experts said, it is a reflection only of what has until now been Americans' relative immunity from terrorism on their own shores.

"Think of how simple it is," said Bruce Hoffman, a terrorism specialist with the Rand Corporation. "Think how many people pull into a garage or park on a street, lock their car and then walk away."

Assassins in the United States have frequently planted bombs in a target's vehicle, including that of the former Foreign Minister Orlando Letelier of Chile who, along with an aide, was killed in a 1976 attack. But Mr. Hoffman and others said the only previous known use of a car bomb in an indiscriminate attack on civilians in the United States occurred in 1973, as part of an attack by Puerto Rican nationalists on five New York banks.

What until now were the two largest terrorist bombings in American history were staged without the help of a car. The most deadly, an explosion in a La Guardia airport terminal, left 11 people dead in 1975. No one claimed responsibility in the attack.

The same year, four people were killed and 65 wounded in the bombing by Puerto Rican separatists of the historic Fraunces Tavern in Lower Manhattan. Even the most recent close call, the arrest in 1988 of a suspected Japanese Red Army member carrying explosives on the New Jersey Turnpike, was believed to have thwarted, not a car bombing, but a more conventional attack.

But as the 1983 assault on a United States Marine barracks in Beirut that killed 241 suggests, the addition of a vehicle to any bombing plot can multiply its destructive power.

Experts say terrorists were initially attracted to car bombs because they could haul heavy loads. Without access to sophisticated explosives, Irish Republican Army terrorists found that an car could carry enough nitrate fertilizer mixed with diesel fuel to create a blast that no hand-delivered explosive could match.

But the experts say the development of potent plastic explosives has added to the threat posed by terrorists, permitting them to pack far more destructive power into a much smaller space. While rarely known to have been used in a car bomb, so-called plastique explosives can explode at a velocity of 23,000 feet per second, many times the rate of ordinary dynamine.

At the same time, terrorists have learned that the seemingly benign shell of a vehicle can allow them to conceal explosives in particularly effective settings. If the aim of whoever planned today's bombing was to create maximum impact, the experts say, the apparent use of the World Trade Center parking garage for a platform is a reminder of the deadly access a vehicle can bring.

For one thing, they said, the enclosed structure of a garage would multiply any explosive's impact, preventing its energy from escaping except at the weakest point, either the walls or the ceiling. Mr. Jenkins of Kroll & Associates said the underground structure would guarantee perhaps more than any other setting that "the explosive force would be more greatly exaggerated."

Perhaps more important, the vehicular access provided to the World Trade Center complex may have provided an opportunity for what other experts said they believed was an attack staged for its impact upon a highly visible American target.

"A car bomb in a street in New York doubtlessly would have killed more people," said Mr. Hoffman of the Rand Corporation, who is the author of several surveys of terrorism in the United States. "But the World Trade Center is a symbol of Wall Street and the Manhattan skyline and the United States itself, and I think that is very important.

"Putting a bomb in a car is not sophisticated," he said. "What's sophisticated about this operation, I would argue, is the planning and calculations that went into it to extract the maximum publicity possible."

After the blast, Federal and local authorities in Washington tightened security around Government buildings and other possible targets in what they described as a precaution against a further attacks. Among Federal officials specializing in explosives, the explosion elicited a sense of awe.

"This is a tremendous blast doing huge amounts of structural damage to steel and concrete," said Jack Killorin, a spokesman in Washington for the Treasury Department's Bureau of Alcohol, Tobacco and Firearms. "I can't overstate this: What went down there was big."

By Robert D. McFadden

The mystery of the explosion that killed five people and injured more than 1,000 others at the World Trade Center in lower Manhattan deepened yesterday as the impact of the apparent bombing on the city's economy and its shaken psyche widened dramatically.

In a move that was expected to deal a heavy blow to Wall Street, officials ordered the sprawling trade center -- with its hundreds of firms, its 55,000 workers and its tens of thousands of visitors daily -- closed indefinitely for repairs and extensive security and safety changes.

As sweeping investigations began, Federal officials said preliminary tests from the debris of Friday's blast found no evidence of plastic explosives often used by terrorists. While the nature of the explosive was still unclear and any definitive interpretation of the tests seemed unwarranted, it appeared that a terrorist's car bomb was only one possible explanation.

Complicating the inquiry, no one claimed responsibility for the attack beforehand, in contrast to the typical behavior of terrorists, although 19 callers made such claims afterward.

Among many avenues of investigation, officials said employees of the Port Authority of New York and New Jersey, which operates the center, were being questioned. And while this was said to be routine, it raised the possibility that a disgruntled or deranged worker might have set off the blast. Security Increased

Security was tightened at hundreds of buildings across New York City yesterday in the wake of the explosion, which destroyed a multistoried parking garage under the trade center, knocked out power to the center and touched off fires that sent smoke billowing up through its 110-story twin towers and forced 50,000 people into a nightmarish evacuation that took all day and half the night.

Two Port Authority workers had not been accounted for yesterday, adding some doubt to the number of fatalities. While it was unknown if the two were at work at the time of the blast, rescue workers searched for them in the morning, climbing on the rubble with devices that detect body heat. They found nothing.

Later in the day, officials concluded that the rubble near the blast site was dangerously unstable and ordered the search ended. After preliminary tests seeking clues to the explosion's source, investigators were also ordered out and told to await a shoring up, which might take days.

Even as authorities assessed the damage, an extraordinary array of public officials -- President Clinton, the Governors of New York and New Jersey, Mayor David N. Dinkins and top law-enforcement officials -- sought to reassure a city shaken by the specter of what many New Yorkers feared was an act of terrorism.

President Clinton, in his weekly radio address, said he had called Gov. Mario M. Cuomo and Mayor Dinkins and pledged "the full measure of Federal law-enforcement resources" for the hunt for those behind the blast.

"Working together, we'll find out who was involved and why this happened," the President declared. "Americans should know we'll do everything in our power to keep them safe in their streets, their offices and their homes."

In New York, the President's message was echoed by state and city officials at an unusual joint news conference that reviewed the death, destruction and turmoil of the explosion, discussed preliminary developments in the inquiries and seemed intended to assure New Yorkers that they were being looked after and that law-enforcement officials were on top of the case.

"Obviously, we will vigorously investigate this callous criminal act, using the resources of all branches of government," said Police Commissioner Raymond W. Kelly. "Beyond that, though, in keeping with the spirit of caution, we will institute a number of measures to safeguard the public." He said the authorities were meeting with security officials of many public and private buildings to tighten security, but he declined to say how or what else was being done.

"It is impossible to speculate at this point why any person should have committed such a horrible crime," Mr. Kelly said. "Nevertheless, the fear it may engender among New Yorkers, and others in our nation for that matter, is very real. We must remember that fear is a type of weapon as well, one to which we must not submit. And we should remember that we are protected by the best law-enforcement personnel -- at the local, state and Federal levels -- in the world."

Asked what he would say to apprehensive and angry New Yorkers, Governor Cuomo acknowledged that the bombing gave people a sense of "being violated," but he added that New York was "still the safest place in the world" and that heightened security would make it "safer still."

Governor Cuomo, Gov. Jim Florio of New Jersey and Mayor Dinkins, who returned yesterday from a weeklong trip to Japan, all toured the scene of the blast yesterday and got a look at a fire-blackened crater, cars twisted like pretzels, windows shattered by office workers gasping for air and other signs of ordeal.

A day after the explosion, there was a host of unanswered questions: Was it a car bomb? Who planted it and why? Was security lax? And what went wrong in the evacuation of more than 50,000 people from the city's largest building complex?

But it was clear from the cautionary tone of official statements that there had been little progress in the investigation.

While stopping short of calling it an act of terrorism or even saying for sure that it was a bomb, law-enforcement officials at the news conference conceded that the explosion that tore out three levels of an underground garage and shook the trade center like an earthquake could hardly have been anything but a bomb.

"In all probability this was a bomb, but we cannot say that with absolute certitude, and that's because we cannot get into the area where the device was exploded," Commissioner Kelly said. Cautious Assessment

In similarly cautious phrases, Mr. Kelly and James M. Fox, assistant director of the F.B.I. in charge of the agency's New York office, said terrorism was a possibility, but they were emphatic in saying it was too early to tell who or what was behind the explosion.

Mr. Kelly reiterated that no one had warned of the bombing in advance, but he said 19 callers to the 911 police emergency number had claimed responsibility in the name of various groups and individuals in the hours after the blast. He declined to give details of those claims.

The first call, he said, was received at 1:35 P.M., more than an hour after the 12:18 P.M. explosion, while another -- a bomb scare that forced the evacuation of the Empire State Building -- came at 2:15 P.M. The rest of the calls were received after 3 P.M. and they continued into the evening hours, he said.

Mr. Kelly, in response to a question, said there had been no unusual activity in the last two weeks that might lead investigators to a particular suspect or group. He said a number of state and Federal vehicles were among those housed in the garage -- even a limousine used by the President when he comes to town -- but he attached no particular significance to this.

Mr. Kelly said it was impossible to know now whether there was more than one device, but he said traces of nitrate had been found in the area of the blast and that this buttressed the theory that the explosion had been caused by a bomb. That theory, he said, was based primarily on the heat, which melted metal, and magnitude of the blast, which left a 100-foot-wide crater that reached three floors down into the bowels of the subterranean garage. No utility equipment, such as a transformer, could cause such an explosion, Mr. Kelly said.

The Commissioner asked anyone with information on the case to call a 24-hour hotline: (212) 577-TIPS. Five Killed

Four men and a woman -- four of them Port Authority workers -- were killed by the blast. The police identified the victims as William Macko, of Bayonne, N.J., in his 40's; Steve Knapp, 48, of Staten Island; Monica Smith, 34, of Seaford, L.I.; Robert Kirkpatrick, 61, of Suffern, N.Y., and John DiGiovanni, 45, of Valley Stream, L.I.

First Deputy Mayor Norman Steisel, filling in for Mr. Dinkins and reviewing the explosion and other developments, said the number of injured had swelled to 1,042. About 15 victims suffered broken bones and other injuries directly in the blast, while the rest suffered smoke inhalation or similar injuries.

Mr. Steisel said there had been a release of asbestos in the explosion, apparently from insulation or fireproofing materials. But environmental officials said there was virtually no danger to trade center employees or rescue workers from asbestos exposure. Air samplings taken at the twin towers were negative, said Albert F. Appleton, the city's Commissioner of Environmental Protection.

Mr. Kelly said there were no security cameras in the basement that might have been helpful in identifying a suspect. Asked if any guests in the Vista Hotel, above the garage, might be under suspicion, he said that that would be a probable avenue of investigation, but gave it no special significance.

Stanley Brezenoff, the executive director of the Port Authority, noted that the blast had knocked out all but three of Consolidated Edison's eight feeder cables to the trade center, cutting most power. He noted that there was a backup generating system, but that became flooded after the blast knocked down water pipes.

Then, as a precaution against danger to rescue workers, the Fire Department asked that all power to the center be cut off. Without power, all lights and 250 elevators were out, forcing some 50,000 people to walk down darkened, smoky stairwells in the twin towers and five other buildings in the complex.

Mr. Brezenoff acknowledged that some doors had been locked for security purposes, and this had prevented people descending stairwells from getting out in some places. But he said calm prevailed for the most part, and he praised rescue workers. Buildings Closed

All but one of the buildings of the trade center -- the exception was Building 7, a relatively small one -- would be closed tomorrow for an indefinite period, Mr. Brezenoff said. Repairs will be made and improvements will be added to security and safety equipment and procedures.

The heavily damaged Vista Hotel, above the blast site, was also closed. Many guests returned yesterday to try to retrieve possessions, but they were not allowed back inside.

"What am I going to do?" said Volerie Voytanka, a Russian tourist whose ticket and passport were in his room and whose flight was scheduled to leave later in the day. Muzak played eerily in the lobby as police officers and guards kept people out.

Mr. Brezenoff said service on the PATH trains from the trade center to New Jersey, shut down by the explosion, may be working again by tomorrow if tests show that tunnels and other structures are sound.

Among the many problems associated with the explosion's aftermath, officials said it might be several weeks before the owners of some 2,000 cars parked in the garage would be able to retrieve them -- provided they have not already been dashed to smithereens.

By Catherine S. Manegold

Police and public safety experts throughout the New York region yesterday drew a tight security net around the city's airports, hotels, major tourist spots and other vulnerable public spaces, moving the city to an alert level not seen since the Persian Gulf war.

The precautions came as preliminary investigations pointed to the possible involvement of a terrorist group in the explosion in the basement of the World Trade Center on Friday.

At Newark International Airport, a short-term parking lot and three roadways were closed, security was increased near plane refueling areas and radar installations and teams of police officers were dispatched to investigate stray luggage and unidentified packages.

At La Guardia Airport, Port Authority police officers dragged away garbage cans and other receptacles that could be used to hide an explosive device and were calling for off-duty officers to come to work. Additional Precautions

The heightened security was visible throughout the city, in closed roads, increased police and security guard presence and extra care taken examining deliveries that on normal days would be waved through into downtown offices, security coordinators at several businesses said.

Explaining that security precautions at the Waldorf-Astoria Hotel in midtown Manhattan were elevated to a level equivalent to that during a fire, flood or electrical interruption, Bill Carroll, the hotel's assistant director of security explained, "We treat these things very seriously. You can't minimize any threat."

The increased security stretched from newspaper offices to subway entrances and even telephone operation centers as the city's residents reacted with fear and astonishment to reports that Friday's explosion in the basement of the World Trade Center may have been the result of a car bomb roughly equivalent in power to the explosion that rocked the United States Embassy in Beirut in 1983. If confirmed as an act of terrorism, the explosion would rank as the most violent terrorist attack in the nation's history, police and other law enforcement experts said yesterday.

While it was unclear what the bomb was made of, terrorism experts described the explosion as having the characteristics of from 200 to 300 pounds of a plastic explosive called Semtex, and said it appeared to have been carefully placed and carefully planned. They differed, however, on whether any terrorist group would claim responsibility and whether the apparent bombing on Friday could be part of a broader plan of terrorist attacks within the United States.

New York City Police Commissioner Raymond W. Kelly said the city was responding to the explosion with one of the largest anti-terrorist operations since the start of the gulf war. At an early afternoon press conference in Manhattan, he and James Fox, chief of the Federal Bureau of Investigation in New York, confirmed that they had no suspects in the case. Mr. Kelly added, however, that police authorities had received at least 19 calls by groups claiming involvement. None came before the midday blast, he said. 'Probably a Bomb'

Though city and state officials stopped short of confirming the blast and fire as an act of terrorism, they ruled out almost any other possibility. "There is an immense crater," said Gov. Mario M. Cuomo. The damage, he said, "looks like a bomb; it smells like a bomb; it's probably a bomb."

In Washington and elsewhere around the nation, other security experts reacted to the news with precautions of their own. Police at the State Department, the Capitol and Union Station in Washington were all placed on heightened alert after the World Trade Center explosion, said Sgt. Dan Nichols of the United States Capitol Police, the force that is responsible for the safety of government buildings including the Capitol, Senate and House of Representatives.

Officials at the White House and at Washington-area airports said their security arrangements were not altered.

William Colby, the former Director of Central Intelligence, said that New York and other cities are in many ways easier to attack than the capital. "You don't just sail into the bottom of the Capitol building, so it is harder to do one of these things in Washington." Ideal Setting for an Attack

New York City, with its crowding, high profile and relatively low security, seemed to have provided an ideal setting for such an attack. "There is just going to be less security at a place like the World Trade Center than at the Congress, the White House or the Supreme Court," said L. Paul Bremer, the managing director of Kissinger Associates in Manhattan and the ambassador at large for counterterrorism from 1986 to 1989. "It is easier to move around in New York, and it is easier to create a great amount of terror there."

What makes such an attack so hard to guard against, said Robert H. Kupperman, a senior adviser at the Center for Strategic and International Studies in Washington, "is that it is easy to do."

"The leverage is always on the terrorist's side because he can go wherever he wants while the government has to deploy ubiquitously," he said.

Across New York City yesterday, that knowledge seemed to be sinking in. Although security officials with the United Nations and many other institutions that could be targets for attacks declined to reveal their security arrangements and any adjustments made to them in the last 24 hours, many institutions explained that they responded as soon as word of the blast spread.

"We've made a minor increase in terms of security in some of our network facilities," said Paula Horii, a public relations representative with A.T.&T. Ms. Horii said the company was preparing to protect computer systems, software, switching stations and other elements of its network.

The airports, however, were where nerves were most frayed. As passengers waited for departing flights at Kennedy, La Guardia and Newark airports, they commented on Friday's explosion and the vulnerability of airline travel. "It shows you that the United States should be more cautious," said Melinda Yearwood, a New Yorker preparing to leave from La Guardia. "But it's not going to change anything for me. I'm still taking my flights."

At Kennedy Airport, Pat Anderson, who had just arrived from Tampa, Fla., and was waiting for a connecting flight to Rome, said she often believes that security is lax. "I have been delayed by increased security before, but I was very pleased," she said. "I think it's worth it."

At Newark International Airport, security was particularly tight. As of 4 A.M. Saturday the complex was closed to all airport staff and ticketed passengers. At about 8:30 A.M., a report of a suspicious package in Terminal B prompted the Port Authority Police to call the Essex County Police Bomb Squad, said Lieut. Robert Campion of the Port Authority police. The squad was sent to check the package and found only a typewriter inside. A corner of the airport terminal, however, was briefly evacuated while the box was examined.

By James Bennet

Port Authority officials acknowledged yesterday that Friday's explosion and the chaotic evacuation that followed it exposed flaws in the World Trade Center's emergency systems. But they defended the existing systems as adequate for almost any emergency, saying the explosion was perfectly, even eerily, situated to cripple the massive complex.

With one blow, the explosion, apparently originating on the second floor of the garage, damaged the complex's emergency command center; pulverized the control that governs it's ventilation system, which could have cleared away the smoke; severed five of the eight main lines carrying power to the twin towers; cracked water pipes, flooding the backup generators in the basement, and cut many of the center's communication lines, making it impossible for many of the workers trapped in the building to call out. Evacuation Criticized

After feeling their way down dozens of flights of steps in smoky darkness without benefit of emergency instructions, many office workers emerged from the towers on Friday criticizing the Port Authority for lacking proper evacuation procedures in its huge complex.

"We learned a few things here," said Stanley Brezenoff, the Port Authority's executive director. Mr. Brezenoff was in the trade center when the blast occurred and negotiated 67 flights to safety.

Mr. Brezenoff said some of the changes that the Port Authority would consider included installing battery-powered emergency lights and communication systems, putting up fluorescent signs and moving the generators that supply backup power in emergencies to a higher floor.

The Port Authority was already planning to put an emergency command center in each of the seven buildings, said Charles Maikish, director of the World Trade Department of the Port Authority.

Port Authority officials repeatedly speculated yesterday whether whoever was behind the bomb believed to have caused Friday's explosion deliberately set it to cripple the complex's emergency systems. "It's certainly no secret," Mr. Brezenoff said of the placement of the systems. "But it's not generally known." 'There Was No Panic'

Despite the breakdown in emergency systems, Mr. Brezenoff emphasized that the evacuation would have taken a long time under any circumstances, and that it was remarkably orderly. "People were calm," he said. "There was no panic."

Officials yesterday also emphasized that, while the blow to vital systems of the complex would shut it down for at least the next couple of days, the explosion caused no structural damage to the two towers.

The only structural threat was to the Vista Hotel, which sits atop the garage where the explosion occurred, said Eugene Fasullo, the Port Authority's chief engineer. The blast collapsed all six levels of the basement, which provided lateral support for the columns supporting the hotel. Engineers were working yesterday to buttress those columns with steel supports, he said. Investigation Suspended

Until those repairs are made and the hotel is secured, Mr. Fasullo said, police investigators are not entering the blast area.

Port Authority officials said that only the building at 7 World Trade Center would be open for business on Monday. PATH train service from the World Trade Center may also resume on Monday, they said.

Another potential hazard posed by the explosion was the release of asbestos fibers, which were embedded in the walls of the garage as fire-resistant material when the complex was built more than 20 years ago, Mr. Maikish said. But he said testing of the air in the lobbies and concourse had not revealed dangerous levels.

"I believe at this point we are O.K.," he said. "We don't have a major asbestos contamination problem."

The World Trade Center is exempt from city fire codes, an exception that Gov. Mario M. Cuomo said yesterday may be reconsidered once Friday's episode has been studied. But Mr. Brezenoff said he knew of no violations of the city's codes at the complex.

"It is true we are exempt, but we make every effort to comply or exceed or have an adequate alternative," he said. Regularly Inspected

He also said that the World Trade Center is regularly inspected by the Fire Department and that he did not think the evacuation on Friday would been any different even if the complex had been governed by the city fire code.

Some observers have suggested that if air in the stairwells had been pressurized, as it is in many buildings, the stairwells would not have acted like chimneys for the smoke on Friday. But Mr. Brezenoff said that the city's fire code regards sprinklers, which the complex has, as an adequate substitute. The ventilation system, he said, would have cleared away the smoke, but the controls governing the ventilation system were destroyed in the explosion.

Although there are reserves for the sprinklers, Mr. Maikish said, the break in the water pipes would have eventually caused them to run dry if there had been extensive fires after the blast. He said he did know how long the reserves would have lasted.

Mr. Maikish said that several years ago a study commissioned by the Port Authority determined that the complex could withstand a car bomb detonated in the garage. But, he said, "I don't think that anybody anticipated the magnitude of this kind of blast."

Mr. Brezenoff said that he could not estimate the cost of repairing all the damage to the World Trade Center. But, he said, the Port Authority has between between $400 million and $600 million of insurance for the complex.