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No one died when the tower, 7 World Trade Center, tumbled, as the estimated 4,000 office workers there at the time had evacuated before it gave way, nearly seven hours after the second of the twin towers came down.
But the collapse of 7 World Trade Center — home at the time to branch offices of the Central Intelligence Agency, the Secret Service and the Giuliani administration’s emergency operations center — is cited in hundreds of Web sites and books as perhaps the most compelling evidence that an insider secretly planted explosives, intentionally destroying the tower.
A separate, preliminary report issued in 2002 by the Federal Emergency Management Agency questioned whether diesel fuel tanks installed in the tower to supply backup generators — including one that powered the Giuliani administration’s emergency “bunker” — might have been to blame.
But S. Shyam Sunder, the lead investigator from the National Institute of Standards and Technology, based here in the suburbs of Washington, also rejected that theory on Thursday, even as he acknowledged that the collapse had been something of a puzzle.
“Our take-home message today is the reason for the collapse of World Trade Center 7 is no longer a mystery,” Dr. Sunder said at a news conference at the institute’s headquarters. “It did not collapse from explosives or fuel oil fires.”
The institute’s findings were released on Thursday as part of a 915-page report resulting from the work of more than 50 federal investigators and a dozen contractors over three years.
Conspiracy theorists have pointed to the fact that the building fell straight down, instead of tumbling, as proof that explosives were used to topple it, as well as to bring down the twin towers. Sixteen percent of the respondents in a Scripps Howard/Ohio University poll said it was very likely or somewhat likely that explosives were planted.
During the last four decades, other towers in New York, Philadelphia and Los Angeles have remained standing through catastrophic blazes that burned out of control for hours because of malfunctioning or nonexistent sprinkler systems. But 7 World Trade Center, which was not struck by a plane, is the first skyscraper in modern times to collapse primarily as a result of a fire. Adding to the suspicion is the fact that in the rush to clean up the site, almost all of the steel remains of the tower were disposed of, leaving investigators in later years with little forensic evidence.
Using videos, photographs and building design documents, the investigators at the National Institute spent the last three years building an elaborate computer model of 7 World Trade Center that they used to test various chains of events to figure out what caused the collapse, Dr. Sunder said.
The investigators determined that debris from the falling twin towers damaged structural columns and ignited fires on at least 10 floors at 7 World Trade Center, which stood about 400 feet north of the twin towers. But the structural damage from the falling debris was not significant enough to threaten the tower’s stability, Dr. Sunder said.
The fires on six of the lower floors burned with particular intensity because the water supply for the sprinkler system had been cut off — the upper floors had a backup water supply — and the Fire Department, devastated by the collapse of the twin towers, stopped trying to fight the blaze.
Normally, fireproofing on a skyscraper should have been sufficient to allow such a blaze to burn itself out and leave the building damaged but still standing. But investigators determined that the heat from the fire caused girders in the steel floor of 7 World Trade Center to expand. As a result, steel beams underneath the floors that provided lateral support for the tower’s structural columns began to buckle or put pressure against the vertical structural columns.
These fires might have been fed partly by the diesel from tanks and a pressurized fuel line, which were on the fifth to the ninth floors, Dr. Sunder said. But the analysis showed that even in the worst case, the diesel fuel-fed fire would not have burned hot enough or long enough to have played a major role in weakening the structure. The investigators determined that the fire that day was fed mainly by office paper and furnishings.