New York's World Trade Center was designed to withstand airplanes crashing into the buildings. They could not survive the devastating fires that followed.
Those fires - fed by thousands of gallons of jet fuel that poured into the towers - burned at temperatures high enough to weaken the steel supports and cause the buildings to collapse.
Once the upper levels starting coming down, their movement caused the lower floors to fall almost straight down on each other, gaining momentum as they went.
Because the building had no columns between the outer walls and the inner core, the concrete floor plates fell on each other like pancakes.
Those are the preliminary findings of structural engineers who have been analyzing Tuesday's collapse - and what might be done to prevent similar failures in the future.
They conclude that no high-rise could have withstood the inferno that engulfed the twin towers after Boeing 767 airliners crashed into them, effectively turning them into giant kilns.
"Nothing would have stopped the World Trade Center from coming down, given the heat of the fire and the length of time it burned," said John Hooper, a principal of Skilling Ward Magnusson Barkshire of Seattle, successor to the firm that served as structural engineer for the World Trade Center towers. "Too much fire for too long a duration will bring any building down."
U.S. building codes are designed to saves lives by giving people time to leave a burning building and extinguish the fire, but codes do not contemplate the sort of conflagration that destroyed the World Trade Center, Hooper said.
"Until now, a fire has never caused this level of catastrophe," he said. "Who could have envisioned something like this? It was a unique and devastating attack that firefighters could not handle."
The towers featured a rigid "hollow tube" structural system made of closely spaced steel columns around the perimeter - 61 to a side - that gave the towers much of their strength.
Floor trusses extended from the perimeter to a central core of steel columns and beams that enclosed stairs and elevators. The design was considered innovative for its time but has since been used in many tall buildings, including Baltimore's tallest, the 40-story Legg Mason tower on Light Street.
The American Society of Civil Engineers is sending forensic teams to the crash sites in New York and at the Pentagon in Virginia to determine in detail the cause of the buildings' collapse and suggest how designs might be improved in the future.
Its study will lead to a definitive report, just as it documented the collapse of the Alfred P. Murrah Federal Office Building in Oklahoma City. But according to Hooper and his colleagues, here is how it likely unfolded:
First, the jet crashes caused fires on the upper levels of each tower, and those fires were accelerated by thousands of gallons of fuel from the airliners, their tanks filled for transcontinental trips that had just begun. The fuel gushing downward caused the fires to spread in the same direction, even as the flames spread upward.
The World Trade Center was one of the first high-rise structures in the United States whose steel columns were treated with a non-asbestos fireproofing material called Cafco Type D, and it had an elaborate sprinkler system. But the force of the plane crashes could have disabled the sprinklers, or they might have been ineffective against jet fuel fires.
Sprinkler systems generally are designed to combat fires on one or two floors at a time. The World Trade Center had eight or more floors on fire at the same time, and the fixed windows turned each floor into an oven. Also, jet fuel fires are typically fought with foam, not water, Hooper noted.
Eventually, the flames broke through the fireproofing material to the steel underneath. Temperatures likely reached as high as 2,000 degrees - and steel can only withstand temperatures from 1,600 to 1,700 degrees before it softens.
On floors affected by the fire, the steel columns and beams bent under the intense heat. Hooper, who did not work on the World Trade Center design, said there is some question as to which failed first, the perimeter columns, the more conventional column-and-beam system used in the buildings' cores, or the trusses and connectors that supported the concrete-on-metal-deck floors.
Engineers in his office initially thought the outer columns might have failed first but later speculated that columns making up the inner core might have collapsed sooner. Hooper said it didn't really matter in the end because once the upper floors starting coming down, their weight caused the ones below to collapse in rapid succession.
A third building, the 47-story office tower known as 7 World Trade Center, collapsed more than seven hours after it started burning - another indication that buildings can't survive lengthy fires.
Worthington, Skilling, Helle and Jackson - the predecessor to Hooper's firm - was the structural engineer for the World Trade Center. Minoru Yamasaki was the lead architect.
Engineers from the firm said eight years ago that the World Trade Center was designed to withstand the impact of a Boeing 707 crash, because they knew a smaller plane had crashed into the Empire State Building. But even then, they warned that it wouldn't be safe from a subsequent fire.
"Our analysis indicated that the biggest problem would be the fact that all the fuel [from the jet] would dump into the building," lead structural engineer John Skilling told The Seattle Times in 1993. "There would be a horrendous fire. A lot of people would be killed."
Skilling's scenario proved to be remarkably prescient.
"We looked at every possible thing we could think of that could happen to the buildings," he told the Times. "However, back in those days, people didn't think about terrorists very much."