Fires in the 47-story office tower at the edge of the World Trade Center site undermined floor beams and a critical structural column, federal investigators [from the National Institute of Standards and Technology] concluded on Thursday, as they attempted to curb still-rampant speculation that explosives caused the building's collapse on Sept. 11, 2001. The collapse of 7 World Trade 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. [Critics] 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. 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. Skeptics ... have long argued that an incendiary material called thermite, made of aluminum powder and a metal oxide, was used to take down the trade center towers. They also have argued that a sulfur residue found at the World Trade Center site is evidence of an inside job.