(AP) -- A federal report on the collapse of the World Trade Center towers highlights flaws in the emergency response and incorrect assumptions about how quickly people can evacuate a skyscraper in crisis, two individuals who worked on the study said Monday.
A team of engineers who have spent more than two years investigating the collapse of the twin towers are to issue three reports Tuesday in New York analyzing the Sept. 11, 2001, building collapses and the response by rescue workers and building occupants.
The National Institute of Standards and Technology will also detail how early decisions played a key role in determining who lived and who died. The attacks killed some 2,749 at the towers, including those who died on the two jetliners that hijackers crashed into the buildings.
The findings represent NIST's last step before issuing its final recommendations in June, the culmination of exhaustive research and testing that produced 10,000 pages of data.
The centerpiece of Tuesday's findings will be the engineers' final conclusions about to the exact sequence of each tower's collapse.
The probe was ordered more than two years ago by Congress to answer lingering questions about the unique design of the World Trade Center buildings, the quality of the buildings' steel, and the ability of the floors and fireproofing to keep them upright.
NIST has already issued preliminary findings that there were no significant problems with the steel.
The two collapses, though different in each building, resulted largely from the way each plane's impact stripped away fireproofing, dislodged key columns, and ignited tons of office material that burned long after the jet fuel had burned away, NIST has concluded.
David Collins, a member of the advisory committee that offered suggestions and questions to NIST investigators, said the research showed design and construction of the building were not major contributors to the collapse.
''I think everyone took deliberate steps to try to do what was necessary to make the buildings as safe as possible,'' said Collins, a Cincinnati-based architect.
The other findings _ about the emergency response and the behavior of those who were in the building _ are likely to fuel an ongoing debate over skyscraper safety.
Investigators have determined that previous expectations about how long people would take to evacuate buildings were not borne out by events at the World Trade Center, according to two individuals who worked on the study and have seen the latest drafts of the reports. They spoke to the Associated Press on condition of anonymity before publication of the data.
The pair who discussed the findings would not specify how much of a time difference the investigators found between evacuation predictions and the actual behavior at the World Trade Center but said the discrepancy often stemmed from individuals' lingering in offices before going to stairwells. That behavior that had not been contemplated in previous models.
The evacuation models are important because architects use them to calculate the capacity needed in stairwells, elevators, and other means of exiting a building.
The report also emphasizes the limited ability of rescue personnel to reach higher floors quickly to battle fires and rescue trapped civilians, the two individuals said.
That proved critical for firefighters who climbed 70 flights of stairs carrying up to 100 pounds of gear _ and then tried to battle flames or clear debris once there.
Those concerns are spurring a debate both within the NIST group and among the larger fire rescue and construction fields about stairwell and elevator design.
The debate centers around whether ''fireproof'' elevators, designed to resist flames and smoke, should be installed in new buildings, particularly those that rise above 40 or 50 stories, and the best width and location of stairwells.
Elevators played a critical, but contradictory, role. In some cases, they helped significant numbers of people get out quickly. For others, they became sealed containers trapping them inside a doomed building.
NIST's ultimate goal is to improve building codes.
