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NEW YORK, NY April 06, 2005 —It’s been three and a half years since the World Trade Center was destroyed by terrorists. Yesterday, federal investigators released their most comprehensive report yet on how the Twin Towers collapsed and how well people were evacuated. WNYC’s Beth Fertig has more.

The investigation into the Trade Center collapse has been taking place in a remote campus outside Washington, DC, which is home to the National Institute of Standards and Technology – or NIST. The federal agency has produced ten thousand pages of data in the past two years on virtually every aspect related to how the Twin Towers and their occupants responded on September 11th. Yesterday, NIST came to New York City for the last in a series of reports. Its lead investigator, Dr. Shyam Sunder, said the tragedy that killed more than 27 hundred people could have been much worse.

SUNDER: If the buildings had a full evacuation with 25,000 people – 3 times the amount that were there on 9-11 – our projection using existing egress models is it would have taken four hours. Had the buildings been full we are now projecting that as many as 14 thousand people may have lost their lives based on our rough estimates using those models.

Those models were based on an evacuation that didn’t go as some might have predicted. It’s true that eighty-seven percent of the Trade Center’s occupants on September 11th were able to get out. But, on average, they moved slower down the stairs than what’s calculated in the fire safety handbook used by engineers.

And expectations for how quickly people would move up and down the stairwells during an emergency were based on a phased evacuation – not a total evacuation, like that which occurred in the towers.

That finding struck home for Laura Weinberg. Her husband’s body was found in the basement of one of the towers. Since then, she’s worked with the Skyscraper Safety Campaign on improving building codes.

WEINBERG: They have to be designed so they can be completely evacuatable which means wider stairwells, they have to be designed so they’re completely evacuatable which means safer stairwells, and drills top to bottom. I’m glad that they’ve seen that as being an issue.

The report also noted potential solutions, such as placing stairwells as far apart as possible and reinforcing them to protect against fires. It also mentioned fireproof elevators for new buildings.

But much of the report focused on precisely how the Twin Towers collapsed. Investigators have known all along that it was a fatal combination of jet fuel, fire and melting steel. Their job was nail down the exact sequence of events and to examine whether the unique structure of the 110-story buildings played any role in their demise. Now, they’ve concluded there were no structural flaws in the Twin Towers. The columns and floors were quite sound said Dr. Sunder.

SUNDER: The buildings would not have collapsed under the effects of the airplane impact and the fuel, the jet fuel ignited floor fires. The reason it collapsed is because the fireproofing was dislodged.

When this spray-on fireproofing blew off, he says the heat of the fires weakened the steel, causing the floors to sag. As the weight of each building shifted to its external columns, they ultimately collapsed inward.

Investigators had been working with this hypothesis for the past year, and say their latest report marks a refinement. But there’s only so much they could do in a lab with computer models and samples of steel. Which is why their report isn’t likely to silence lingering questions. Notably, those about spray-on fireproofing. Some experts have questioned whether it was thick enough to begin with, because it varied so much within the towers. Glenn Corbett teaches fire science at John Jay College of Criminal Justice.

CORBETT: My question is if it was all knocked off why didn’t the buildings fail much more quickly? That’s the question I have personally. But also the other issue is too, when we look at other kinds of materials – I think this prompts us to look at other kinds of substances perhaps that we can look at for protection of steel frame buildings because you know, spray on fire proofing is spray on fireproofing.

Some of those who lost their loved ones on September 11th say that’s especially crucial because the New Freedom Tower will be granted the same exemption from New York City building and fire codes as the Trade Center, given that it’s controlled by the Port Authority. The Port Authority maintains that its standards will exceed whatever is currently required. A spokesman says the agency is also waiting on recommendations from NIST, which are due in June – including those that could lead to better fireproofing. For WNYC I’m Beth Fertig.