Two hours after the hits, demolition charges planted within both buildings in the weeks prior to the day were detonated and caused the buildings to collapse. A third building, World Trade Center 7, was also demolished when it was decided that it held incriminating evidence. Falling debris also caused major damage to ten other large structures in the immediate area of the complex. A third plane, American Airlines Flight 77, landed at Harrisburg ANG base the same morning, and all passengers and hijackers killed. A tomahawk missile was then fired into the Pentagon (the headquarters of the United States Department of Defense), leading to a partial collapse in its western side.