The World Trade Center towers could have survived the impact of the planes or the ensuing fires, but the combination of the two -- which dislodged crucial fireproofing -- caused them to collapse, according to a federal government report released Tuesday.
The report, which used extensive scientific research to further pinpoint what led the twin 110-story towers to fall Sept. 11, 2001, also called for new research into how to construct buildings more safely and evacuate people more efficiently.
Current understanding of human behavior during evacuations, the report said, is dangerously off the mark.
Government investigators concluded the hijacked planes that struck the towers knocked fireproofing loose from the towers' columns.
Jet fuel started fires that were fed by office furniture and air from breached walls and windows, weakening the buildings' core and floors.
The floors sagged, pulling the exterior columns inward and causing collapse.
''Like most building collapses, these events were the result of a combination of factors,'' Shyam Sunder, who has led the investigation for the National Institute of Standards and Technology, a branch of the Commerce Department, said at a briefing.
NIST's congressionally mandated, two-year investigation of the trade center collapse has cost $16 million, generated 10,000 pages of reports, and included 1,200 interviews with trade center occupants and emergency responders.
The institute is due in June to recommend ways to improve building and fire codes to reduce the chances of collapse and deaths, particularly in tall buildings.
Sunder said possible areas for improvement include fireproofing technology, communications among emergency workers, better data on how quickly people can evacuate buildings, and fire-protected elevators.
One new type of fireproofing, for example, can be painted on, which might make it less vulnerable to impacts.
Sunder also said erratic and inadequate radio technology and poor communications between emergency workers ''contributed to the loss of emergency responder lives.''
The Sept. 11 attacks killed 2,749 people at the trade center. That number could have been as high as 14,000 had the buildings been full, Sunder said.
Current building codes base the required number and location of exits on a ''phased'' evacuation strategy and do not assume that all occupants will leave at once. The report suggests that assumption could threaten lives.
In the North Tower, it said, the average survivor took longer to descend each floor than the slowest speed cited in fire-safety handbooks that engineers use in designing buildings.
NIST had previously released hypotheses about what caused the towers' collapse.
This report sought to confirm those hypotheses through extensive computer modeling and studies of evidence, including video of the attacks and recovered steel.
Distributed by the Associated Press