Smoldering debris from one of the fallen Twin Towers ignited the nearby World Trade Center Building 7, and the intense heat -- not explosives -- caused the skyscraper to collapse, according to a federal report released Thursday.
The findings resulted from a three-year investigation by the National Institute of Standards and Technology. Though various experts have long believed fire played a role in the building's destruction, the institute's investigators said it was the primary cause and the "first known instance of fire causing the total collapse of a tall building."
"Obviously, designers and engineers will be thinking of their buildings as they consider our report, and they'll take appropriate action," said lead investigator Shyam Sunder.
Guidelines for skyscraper construction here and across the globe were revamped after the Sept. 11, 2001, attacks.
Critics questioned why the NIST investigation took so long, saying that there were signs early on pointing to a fire-related collapse.
James Quintiere, a professor of fire protection engineering at the University of Maryland, wondered how the institute was able to definitively rule out explosives.
"They don't have the expertise on explosives, so I don't know how they came to that conclusion," said Quintiere, a frequent critic of the agency, where he formerly worked as chief of the fire science and engineering division.
Quintiere stressed, however, that he had never believed explosives played a role.
After the World Trade Center's North Tower fell at 10:29 a.m., debris sparked fires at Building 7, which was 370 feet south. Building 7 burned for several hours.
Water supply lines for the building's automatic sprinkler system were cut off by the collapse of the Twin Towers, worsening fire conditions.
Heat from uncontrolled flames caused thermal expansion of steel beams, the report said.
When the beams expanded, they pushed supportive beams and damaged flooring surrounding columns.
Finally, a support column buckled, triggering an "upward progression of floor system failure," the report said.
The scientific investigation considered other credible possible causes of the building's collapse, he said, including explosives and a stored diesel-fuel supply for backup generators.
"Even from the beginning, we did not have any evidence at all that demolition or charges or a blast were used to bring the building down," Sunder said.