The al-Qaeda organisation under Osama bin Laden had been conducting attacks against American interests since the 1998 American embassy bombings in Kenya and Tanzania (224 dead) and the 2000 attack on the USS Cole in Aden (17 dead). The 11 September 2001 operation had been in planning since approximately 1998.
The operational concept — conceived by the Pakistani militant Khalid Sheikh Mohammed — was the simultaneous hijacking of commercial aircraft for use as cruise-missile weapons against American symbolic targets. Approximately 19 hijackers were recruited from Saudi Arabia (15), the United Arab Emirates (2), Lebanon (1), and Egypt (1). They entered the United States across 2000-2001 and attended flight training schools in Florida, Arizona, and Minnesota.
The morning
Four American commercial flights were hijacked between 8:14 a.m. and 9:28 a.m. on Tuesday 11 September 2001:
— American Airlines Flight 11 (Boston-Los Angeles, hijacked 8:14 a.m., 92 people aboard) — United Airlines Flight 175 (Boston-Los Angeles, hijacked 8:42 a.m., 65 people aboard) — American Airlines Flight 77 (Washington Dulles-Los Angeles, hijacked 8:51 a.m., 64 people aboard) — United Airlines Flight 93 (Newark-San Francisco, hijacked 9:28 a.m., 44 people aboard)
The hijackings used knife threats and bomb-threat bluffs. The post-1970s American hijacking protocol — based on earlier hijacking patterns — had been to cooperate with hijacker demands and land the aircraft at the demanded destination. The pilots and flight attendants of the first three flights followed the protocol. The protocol had not anticipated suicide-attack use of the aircraft.
The towers
Flight 11 struck the North Tower of the World Trade Center at 8:46:30 a.m. at approximately 750 km/h between the 93rd and 99th floors. The jet fuel ignited and burned at approximately 1,000°C across the impacted floors. Substantial all people above the impact zone — approximately 1,344 people — were trapped above the fire.
Flight 175 struck the South Tower at 9:03:11 a.m. at approximately 950 km/h between the 77th and 85th floors. The South Tower impact happened on live broadcast television — the first impact had been seen only by New York witnesses.
Flight 77 struck the west face of the Pentagon at 9:37:46 a.m. at approximately 850 km/h. Substantial 125 Pentagon employees and military personnel died, plus 64 aboard the aircraft.
Flight 93 never reached its intended target — probably the United States Capitol or the White House. The passengers had learned about the other hijackings via in-flight phone calls and had decided to resist. The cockpit voice recorder recorded the passenger assault on the cockpit at approximately 10:03:11 a.m. The hijackers deliberately crashed the aircraft into a field near Shanksville, Pennsylvania, rather than be boarded. All 44 aboard died.