Four passenger airliners operated by two major U.S. passenger air carriers ( United Airlines and American Airlines ) — all of which departed from airports in the northeastern United States bound for San Francisco and Los Angeles — were hijacked by 19 al-Qaeda terrorists . Two of the planes , American Airlines Flight 11 and United Airlines Flight 175 , were crashed into the North and South towers , respectively , of the World Trade Center complex in Lower Manhattan . Within an hour and 42 minutes , both 110-story towers collapsed . Debris and the resulting fires caused a partial or complete collapse of all other buildings in the World Trade Center complex , including the 47-story 7 World Trade Center tower , as well as significant damage to ten other large surrounding structures . A third plane , American Airlines Flight 77 , was crashed into the Pentagon ( the headquarters of the U.S. Department of Defense ) in Arlington County , Virginia , which led to a partial collapse of the building 's west side . The fourth plane , United Airlines Flight 93 , was initially flown toward Washington , D.C. , but crashed into a field in Stonycreek Township near Shanksville , Pennsylvania , after its passengers thwarted the hijackers . 9/11 is the single deadliest terrorist attack in human history and the single deadliest incident for firefighters and law enforcement officers in the history of the United States , with 343 and 72 killed , respectively
United Airlines Flight 175
United Airlines Flight 175
September 11 attacks