June 18 (Bloomberg) -- Federal engineering investigators studying the destruction of the World Trade Center's twin towers on Sept. 11 said New York Police Department aviation units reported an inward bowing of the buildings' columns in the minutes before they collapsed, a signal they were about to fall.

``No evidence has been found to suggest the information was communicated to all emergency responders at the scene,'' said an executive summary of a progress report by the National Institute of Standards and Technology, which is conducting the study.

Investigators also found evidence that the south tower, which fell 56 minutes after it was struck by terrorists in a hijacked commercial jet, had less fire proofing than the north tower, which held up for about double the time after it was hit. The buildings were about a third full when the planes struck, and had they been fully occupied with 50,000 people, a complete evacuation would have taken four hours, probers found.

The institute, a U.S. government agency whose mission includes developing office-building safety standards, is almost two years into an investigation of the 2001 trade center attack, which killed 2,749. Its findings follow reports from a federal commission studying the attacks that cited lack of coordination among emergency workers in their response at the site.

``The NYPD aviation unit reported critical information about the pending collapse of the building,'' said Sivaraj Shyam- Sunder, who heads the institute, at a press briefing in New York. ``Any time that information could have been communicated faster to the emergency responders in the buildings, it would have helped save lives.''

No Recommendations

The institute, which also held a presentation for the public on its progress which was attended by family members of victims, won't make any recommendations on building safety until it completes its study and releases it to the public next year, Shyam-Sunder said.

The probe has included reviews of audio-taped interviews with police, fire and other emergency personnel who entered the buildings that day, interviews with attack survivors and technical studies of the towers' structure and evacuation routes. Investigators have also built and shaken or burned models of trade center floors to determine how they came apart.

Buckling Steel

According to Shyam-Sunder, the concave bowing of the steel was seen on the sides of the towers opposite where the planes hit them. At 10:06 a.m. that morning, an officer in a police helicopter reported that ``it's not going to take long before the north tower comes down.'' This was 20 minutes before it collapsed. In another radio transmission at 10:21 a.m., the officer said he saw buckling in the north tower's southern face, Shyam-Sunder said.

The report includes photographs taken from police helicopters showing the bending columns.

Police had already ordered a complete evacuation of the north tower at the time those transmissions were made, said Police Department Inspector Michael Coan. Both transmissions came after the south tower was already down, he said.

The police and fire departments have since upgraded their communications, said Frank Gribbon, deputy fire commissioner. Batallion chiefs are now authorized to ride in police helicopters for high-rise fires, and firefighters may also be deployed to rooftops via police helicopters, when such an action is found to be safe, he said. The two departments have cooperated in a helicopter drill, Gribbon said.

As to what happened on Sept. 11, he said, ``I can't reconstruct that day.''

Fireproofing

Investigators found that the south tower had 0.6 inch of fireproofing, on average, sprayed onto its structural steel members, compared with the north tower's 1.5 inches, Shyam-Sunder said. The Port Authority of New York and New Jersey, which owned the towers, was in the midst of applying fresh protectant to the entire complex at the time of the attacks, he said.

Coincidentally, in June 2001 an international group had recommended a minimum coating of 2 inches for floor systems similar to the trade center's, to achieve protection through two hours of burning, Shyam-Sunder said.

The disclosures come amid scrutiny of the nation's and city's preparedness for and response to the Sept. 11 attacks by the federal commission studying the deadliest assault on U.S. soil. A report released by the panel this week said the U.S. military was neither trained nor prepared to respond to suicide attacks by hijacked planes when those attacks occurred.

`A Disgrace'

Sally Regenhard, chairwoman of the Skyscraper Safety Campaign and mother of probationary firefighter Christian Regenhard, who was killed on Sept. 11, called the institute's findings ``a disgrace,'' and further evidence that firefighters were left incapable of saving themselves once the buildings were deemed unsafe. Most upsetting, she said, were findings from the institute's report that the towers' fire-protected steel frame was rated to last two hours, at best.

``That means I'm putting you into a building that in case of an emergency and a fire like this, you won't get out,'' Regenhard said. ``These are deeply disturbing things.''

To contact the reporter on this story: David M. Levitt in New York at dlevitt@bloomberg.net.

Last Updated: June 18, 2004 17:55 EDT