Al-Qaeda executed the September 11 attacks.
American Airlines Flight 77 . The Boeing 757-223 aircraft serving the flight was hijacked by five men affiliated with al-Qaeda on September 11 , 2001 , as part of the September 11 attacks . September 11 attacks . A third plane , American Airlines Flight 77 , was crashed into the Pentagon ( the headquarters of the United States Department of Defense ) in Arlington County , Virginia , leading to a partial collapse of the building 's western side .