DEVLIN BARRETT, AP

GAITHERSBURG, Md. (Aug. 21) - Federal investigators said Thursday 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. On Sept. 11, it was set on fire by falling debris from the burning towers, but skeptics have long argued that fire and debris alone should not have brought down such a big steel-and-concrete structure.

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