A two-year study is investigating the towers' collapse
The floors of the World Trade Center had not been tested for their ability to withstand fire, an interim report has found.
Extreme heat generated by the many fires that broke out after two aircraft were flown into the buildings on 11 September is seen as one potential factor in their collapse.
The government agency that built the towers never tested the fireproofing insulation on the WTC's floors - meant to withstand two hours of fire - according to the National Institute of Standards and Technology.
Arden Bement, NIST director, said the report did not draw conclusions or make recommendations.
"That's for later, when we have a much more complete picture," he said.
The NIST's two-year investigation was launched last year, and will analyse the collapse and evacuation of the WTC in an effort to make buildings safer in the future.
Despite the widely accepted notion that no skyscraper could have withstood a fuel-laden jet slicing into its core, relatives say there are still many technical questions to be answered.
Previous reports have found that the towers showed "remarkable resilience", standing for more than an hour after being hit by the aircraft.
But some experts say the buildings may even have withstood the strikes, had fires not weakened their support structures.
In 1999, the building's owners, the Port Authority of New York and New Jersey, issued guidelines to upgrade the fireproofing by tripling the thickness on the flooring.
As recently as 2000, officials were still redoing the fireproofing, noting in one property assessment that some areas could withstand only one hour in a fire.
By 11 September 2001, the NIST report found, fireproofing had been upgraded on 29 floors in the areas of the towers where the hijacked planes struck.
Glenn Corbett, a US fire science expert, was quoted by AP saying that the upgraded system should have been tested.
But the fast-moving fires in the towers on 11 September may have been far more severe than the typical conditions assumed in the fire resistance test, he added.