GAITHERSBURG, MD. - Federal investigators said Thursday that they have solved a mystery of the Sept. 11, 2001, attacks: the collapse of World Trade Center building 7, a source of long-running conspiracy theories.

The 47-story trapezoid-shaped building sat north of the World Trade Center towers, across Vesey Street in lower Manhattan in New York.

On Sept. 11, it was set on fire by falling debris from the burning towers, but skeptics long have argued that fire and debris alone should not have brought down such a big steel-and-concrete structure.

Scientists with the National Institute of Standards and Technology say their three-year investigation of the collapse determined that the demise of WTC 7 was the first time in the world a fire caused the total failure of a modern skyscraper.

"The reason for the collapse of World Trade Center 7 is no longer a mystery," said Dr. Shyam Sunder, the lead investigator on the NIST team.

Investigators also concluded that the collapse of the nearby towers broke the city water main, leaving the sprinkler system in the bottom half of the building without water.

The building has been the subject of a wide range of conspiracy theories for the last seven years, partly because the collapse occurred about seven hours after the twin towers came down.

That fueled suspicion that someone intentionally blew up the building in a controlled demolition.

Mike Berger of the group 9/11 Truth said he wasn't buying the government's explanation.

"Their explanation simply isn't sufficient. We're being lied to," he said, arguing that there is other evidence suggesting explosives were used on the building.

Sunder said his team investigated the possibility that an explosion inside the building brought it down but found there was no large boom or other noise that would have occurred with such a detonation.

The 77-page report concluded that the fatal blow to the building came when the 13th floor collapsed, weakening a critical steel support column and leading to catastrophic failure.

"When this critical column buckled due to lack of floor supports, it was the first domino in the chain," said Sunder.

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For more information about the investigation, go to www.nist.gov/.)

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