The findings by the institute, however, still do not exonerate Mr. Robertson or the building's owner, the Port Authority of New York and New Jersey, which, in defending the trade center project from critics in the 1960s, boasted that the design was so robust that the towers could be hit by a jet traveling at 600 miles per hour without collapsing or endangering the lives of occupants beyond the impact zone. In retrospect, such a claim was unjustified because the engineers had failed to consider the added stresses caused by the resulting fires.

Sally Regenhard, who founded a group called the Skyscraper Safety Campaign in honor of her son, Christian Regenhard, a probationary firefighter who was one of 2,749 victims, said she was not ready to concede that the towers' design were not fundamentally at fault.

"It is far, far too premature to come to any conclusions that it wasn't the fault of the building, or nothing was wrong with the building," she said after listening to a daylong presentation in Gaithersburg, Md., about the latest findings.

The investigators have examined just about every possible factor that could have contributed to the collapse, including the steel used in the columns. Computer models were used to calculate, as accurately as possible, where different airplane parts traveled, and what kind of damage they did. Then, intricate models were built, essentially recreating the resulting fires.

Through all this, particular attention has been focused on the innovative floors that were central to the design of the twin towers. The floors were particularly critical in the trade center because in office buildings built before the 1960's, structural columns and beams were generally spread throughout -- holding up the enormous weight and allowing the tower to resist the force from wind.

In the trade center, only the building's exterior and core had structural columns, and in between them were wide-open floors -- relatively lightweight, decklike structures. Some engineers have wondered if insufficient fireproofing on the floor trusses led them to fail, undermining the structural integrity of the towers.

The federal investigators found, after conducting a test with a reconstructed section of the floor, that the original fireproofing on the floors, as built, was sufficient to ensure that they met the New York City building code under standard testing parameters.