During planning for the World Trade Center in the 1960s, structural engineers designed the twin towers to withstand the worst storm in 150 years. As an afterthought, the project engineer said, they calculated that the skyscrapers could sustain a direct hit from a 124-ton Boeing 707.
But the engineer concedes that they did not take into account a critically important ingredient in an airplane crash: volatile jet fuel.
That oversight bred false confidence among the trade center's managers, the New York City Fire Department and other emergency agencies. Had officials been aware that the towers were not designed to remain standing in the face of jet fuel burning at 2,000 degrees, a series of miscalculations might have been avoided:
* The Fire Department's emergency command post was set up directly beneath the twin towers, which collapsed and killed the department's chief, chaplain and other firefighters.
* Workers on some lower floors of both towers were not told to evacuate immediately, and instead remained inside during the crucial minutes before each tower fell.
* The Fire Department's top structural expert rushed into the trade center in a desperate--and ultimately fatal--effort to assess the skyscrapers' structural viability as the flames spread.
* And firefighters raced into the towers unaware that they had a short window of time--less than an hour in one building and less than two hours in the other--in which to evacuate thousands of workers and get themselves, police and other rescuers to safety.
The center's south tower toppled 52 minutes after it was struck by United Flight 175 on Sept. 11. The north tower, which was struck first, went down an hour and 43 minutes after American Airlines Flight 11 sliced into the building.
The sight of those towers crumbling so quickly was never anticipated by generations of trade center officials and Fire Department commanders, who were assured for years by engineers that the towers could stand up to a crashing Boeing 707.
"Our guys stayed in there so long because they didn't think the building would come down," said Fire Lt. Andrew Graf of Engine No. 4 and Ladder No. 5, from which 14 firefighters are missing and feared dead.
It was not until the south tower began collapsing that his men were ordered to turn around and get out of the north tower, Graf said. That tower came tumbling down with firefighters inside just 28 minutes after the first collapse.
"I'm sure it was realized after the building burned for a while that the steel was going to get weak," said Lt. Chris Piazza of Engine No. 24 on Sixth Avenue, which lost 11 firefighters. "The chiefs on the scene had called for evacuation before the collapse, but a building of that size takes a long time."
Firefighters say they were never trained to fight a building fire caused by an airplane crash. Their drills never included training on how to handle burning jet fuel inside a skyscraper, they said.
"Everyone's fears were focused on a chemical attack or a bomb," Piazza said.
A former top city official familiar with emergency procedures said there was "some talk" among contingency planners about a small private plane accidentally striking the towers, but that possibility never was translated into the Fire Department's training.
Hyman M. Brown, the project engineer for the twin towers, said that in designing the trade center in the 1960s, "we thought of the structural impact of a plane hitting, but not the burning fuel." The towers withstood the tremendous structural impact of the two hijacked airliners as they slammed into the buildings on the morning of Sept. 11, Brown said.
The towers' supporting columns were made of steel, which melts at 1,500 degrees, Brown said, or about 500 degrees lower than the heat of the burning fuel. The searing heat caused the columns to buckle and sag, triggering the collapses.
At least 35 firefighters have been confirmed dead as a result of the attack, and more than 300 are among the missing.
A Fire Department spokesman did not respond to questions faxed to department headquarters Wednesday. A spokesman for the firefighters' union referred all questions to the department.
Officials have said they would not have set up their emergency command post at the foot of the towers had they realized that a jet fuel fire was capable of bringing down the skyscrapers so swiftly. They were accustomed to high-rise fires, which, no matter how extensive, normally leave the buildings standing.
Richard Sheirer, director of the mayor's Office of Emergency Management, said officials decided to move the command post across the street to the World Financial Center after the second tower was struck at 9:08 a.m.
Ray Downey, chief of the special operations command and the Fire Department's top expert on building collapses, ran into the World Trade Center complex after the first plane struck. As he attempted to determine the stability of the structures, the south tower collapsed.
Firefighters interviewed Wednesday stressed that even if they had known about the possibility of collapse, they still would have rushed into the towers to save people. They declined to speculate what, if anything, they would have done differently.
"If you know there's life in there, you go inside," Graf said. "That's what they're paying us for."
As early as the 1980s, some officials of the Port Authority of New York and New Jersey talked among themselves about the possibility of a terrorists' plane striking the trade center complex, according to a former authority official. The authority is responsible for emergency procedures at the complex.
The official, who asked not to be identified, said the authority envisioned a small private plane carrying explosives--not a massive passenger jetliner piloted by terrorists and filled with hostages. In any case, he said, those concerns were not followed up on or incorporated into emergency training procedures.
The original assurances by the trade center's engineers that the towers would remain sound even if struck by a Boeing 707 led a series of Port Authority officials to assume they were safe from collapse.
"It was built to withstand the impact from a plane. The building models took that into account," said Stanley Brezenoff, who was executive director of the Port Authority when terrorists detonated a truck bomb at the trade center on Feb. 26, 1993. Six people were killed and more than 1,000 were injured after a bomb in a rented truck exploded in an underground parking garage.
Attempting to tighten security after that attack, the Port Authority commissioned studies by private security consultants. Those studies focused primarily on protecting the complex from a ground attack, not an air assault.
Structural engineers say it is possible to design and build a skyscraper able to withstand a jetliner crash involving burning fuel. But such a structure would be prohibitively expensive for tenants and would look and feel like a bunker.
Garry Briese, executive director of the International Assn. of Fire Chiefs, said few, if any, fire departments in the U.S. have plans for fighting a fire caused by an airliner crashing into a skyscraper.
"It's such an infrequent occurrence all the way around," Briese said. "All bets are off right now as far as what people are going to be looking at" in future planning.
Fire officials in Los Angeles, Chicago and Miami said Wednesday that their departments do not train specifically for disasters involving passenger planes slamming into skyscrapers.
"We assume that any building in L.A. hit by a jetliner would fall in a heap, just like the towers in New York," said Bob Kholos, spokesman for the Los Angeles Fire Department.
Times researcher Lianne Hart and staff writer Charles Ornstein in New York and Ken Reich in Los Angeles contributed to this story.