That the buildings remained standing as long as they did was a tribute to their design and construction, according to the report from the Federal Emergency Management Agency and the American Society of Civil Engineers.
"The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable and is the reason that most building occupants were able to evacuate safely," the report stated.
It suggested the trade center's unique steel supports — called trusses — "may have played a role in allowing the buildings to collapse in the manner that they did." But the report said more study was needed before a final conclusion could be drawn.
Lawmakers at the Science Committee hearing where the report was formally unveiled Wednesday said additional study was needed to make buildings safer.
"If we want to ensure that the legacy of this tragedy is that future building collapses are avoided or mitigated, we need to do a better job of investigating the causes," said Rep. Anthony Weiner, a New York Democrat.
Before it was presented, the report was lambasted at a news conference by a group representing relatives of people who died in the attack.
"This was not an investigation and we want to know what happened, why it happened and make sure it never happens again," said Sally Regenhard, whose New York firefighter son Christian died trying to rescue people in the attack.
Rep. Weiner said the study was "like the Keystone Kops," referring to the bumbling policemen depicted in silent movies.
"It was like investigating the murder but not having the weapon, not having fingerprints and not having access to the crime scene," said Weiner, who is pushing for a federal investigation.
The expert panel recommended improvements to choose codes, including fireproofing that sticks to steel beams and emergency stairwells "hardened" to withstand the catastrophic impact of a plane in future high-rise buildings.
Investigators suggested improving aviation security rather than trying to harden buildings against airplane impact.
The engineers said studies will continue, but said the buildings collapsed because of three factors: the force of the aircraft impacts; the heat from burning jet fuel; and the heat from the burning of much of the buildings' contents.
The report said fire systems are not designed to deal with blazes that erupt instantly across several acre-sized floors, as happened when thousands of gallons of fuel from the hijacked planes exploded.
Instead such systems envision fires starting small and spreading slowly, with sprinklers and firefighters working together to limit the damage.
The report said the World Trade Center buildings could still be standing despite the hits by the jets if not for the additional stress caused by the fires.
"The large quantity of jet fuel carried by each aircraft ignited upon impact," the report said, but concluded that "the heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses."
It said as the burning fuel spread across several floors, it ignited the buildings' contents, such as computer terminals, carpets and furnishings, producing heat "estimated to have been comparable to the heat produced by a large commercial power-generating station."
It said these fires unevenly heated the steel structures, which "induced additional stresses ... while simultaneously softening and weakening" the steel.
The report said it was unclear whether engineering ever could protect buildings from fast-moving aircraft. "Reliably designing a building to survive the impact of the largest aircraft available now or in the future may not be possible," the report said.
Worcester Polytechnic Institute Professor Jonathan Barnett, one of the study's investigators, said the study proved overall the trade center performed well on Sept. 11.
"We didn't find a glaring blunder," Barnett said.