Federal investigators said Tuesday the World Trade Center (search) buildings probably would not have collapsed in the Sept. 11, 2001, attacks if fireproofing had adhered firmly to the columns and floors.

The National Institute of Standards and Technology (search) also concluded that the average survivor took more than double the estimated time to descend emergency stairwells, and that better communication between emergency responders could have saved more lives.

The NIST, which issued three reports on the attacks, did not blame designers or builders for the buildings' collapse. However, Shyam Sunder, who led NIST's fire and safety investigation, said there are now better ways to ensure that fireproofing adheres to steel.

"Even with the airplane impact and jet-fuel-ignited multi-floor fires, which are not normal building fires, the buildings would likely not have collapsed had it not been for the fireproofing that had been dislodged," he said.

The reports are likely to spur debate about how to build safer skyscrapers. Some investigators and rescuers have advocated "fireproof" elevators and stronger stairwells in new high-rises. In theory, fireproof elevators could have taken firefighters to the upper floors and helped people get out faster.

The NIST findings were met with mixture of skepticism and praise from family members of Sept. 11 victims who attended a public hearing on the reports.

Laura Weinberg, whose husband Richard Aronow died in the attack, said she was disappointed with some of the conclusions about design features. "I think they've soft-pedaled the issue of spray-on fireproofing," she said.

But Allan Horwitz, whose son Aaron also died in the attacks, said he was "very pleased" with NIST's work. "It seems to me like they're doing everything they can to find out what happened, why it happened, and how we can prevent it from happening in the future," he said.

The report also found that in Tower 1, it took the average survivor 48 seconds to descend a flight of stairs. This was double the slowest evacuation time estimated in a current fire safety handbook used by engineers who design buildings, the report said.

The report said that some people delayed their evacuation by "milling" in offices, deliberating about what to do, or debating how to find a stairwell.

NIST said the times in the models are based on phased evacuations, not the full-scale evacuation that occurred in the towers. The evacuation models cited in the NIST report are used by architects to calculate how much capacity is needed in stairwells, elevators, and other exit routes.

In addition, the report found that the much-documented problems with radio communication and information-sharing among first responders probably "contributed to the loss of emergency responder lives."

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 official World Trade Center death toll stands at 2,749, including those killed on the two jetliners that hijackers crashed into the buildings.