The hijackers chose transcontinental flights, which carry heavy fuel loads, and passed through checkpoints in Portland, Boston, Dulles and Newark. A computer prescreening system called CAPPS flagged several of them, including Atta, but under the rules then in force the only consequence was that their checked bags were held until they boarded. [1]
In each case the hijackers waited until the plane was at cruising altitude, then forced their way into the cockpit, killed or removed the pilots, and turned off or changed the transponder. Flight attendants and passengers phoned airline offices and relatives with descriptions of knives, Mace or pepper spray and claims of a bomb. [1]
United 93, which left Newark late and carried only 37 passengers including the four hijackers, was the only plane where the people aboard learned what had happened in New York before their own hijackers struck. Their revolt began at 9:57; at 10:03:11 the plane hit the ground at about 563 mph, upside down. [1][2] Both towers collapsed within an hour and three quarters of being hit. [3] The individual pieces are in the towers file, the Pentagon file, the Flight 93 file and the air-defense file.
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