Investigators believe inadequate fireproofing of steel beams may explain why the twin towers in New York collapsed so rapidly after they were hit by hijacked airliners on 11 September.
A panel of engineering and fire protection experts hired by the US government to look into the problem has heard how fire-insulation covering had a history of peeling off the buildings' beams and joists.
Frederick Mowrer, a fire protection professor at the University of Maryland, suggested that steel beams holding up floors may have softened and then given way in the intense fires. Their giving way may have started a domino effect.
He said: "It seems that the fireproofing was not up to what it should have been."
Abolhassan Astaneh-asl, an engineering professor at the University of California in Berkeley, agreed. He has been analysing the damage the aircraft had caused when they struck the towers.
He calculated that they completely severed about 40 per cent of the supporting columns and even penetrated to the columns in the core of each tower. "It's like a bullet hole," he said, but added that without the fires, the buildings should have remained standing.
In the only two comparable incidents in America, skyscrapers suffered devastating fires but stayed upright. In 1988 five floors of the 62-storey First Interstate Bank building in Los Angeles burnt for three and a half hours. Three years later, the upper nine floors of Philadelphia's One Meridian Plaza burnt until sprinklers extinguished the flames.Reuse content