More than two years ago, the National Institute of Standards and Technology, N.I.S.T, began a detailed study of the twin towers' collapse, looking for ways to improve building structure, fireproofing and evacuation plans for the future.

The deputy director of the N.I.S.T, Shyam Sunder, briefed reporters on the progress of the investigation (Friday). He said horizontal floor beams in the buildings had less fireproofing protection than the columns that supported the towers, and stronger fireproofing might have delayed the collapse.

"If a component is essential to the stability of the building, it should be fireproofed to the same fire-rating as the columns in the building which usually have a higher fire-rating. If that concept had been used, and by the way that concept was not required by New York City building code, it would have required the floor truss system to have had a one hour greater fire resistance equal to that of the columns," he said.

He also said that tests for how the enormous building could withstand the force of wind were inadequate during the initial design stage of the World Trade Center towers. Recent wind tests showed that the potential force of gusts could be 40 to 60 percent higher than the original designers anticipated.

The N.I.S.T. also released the first official estimate of how many people were in the buildings when the planes struck on September 11, 2001, placing the figure at between 16,200 and 18,600 people. New York City officials had estimated that as many as 25,000 people were in the buildings.