Maryland ? Federal investigators said Thursday in Gaithersburg 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 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 the demise of WTC 7 was the first time in the world a fire caused the total failure of a modern skyscraper.

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.