Understanding 9/11

Of the four planes involved in the Sept. 11 attacks, three were hijacked by five-member teams of terrorists. But United Airlines Flight 93, which crashed in Pennsylvania, was seized by just four. Since then, there has been endless speculation as to who may have been the intended 20th hijacker—who somehow never made it onto the plane. The 9/11 commission determined that, besides the 19 hijackers, eight other al-Qaeda members for various reasons failed to enter the U.S. to take part in the attacks. One was Detainee 063, Mohammed al-Qahtani, who investigators strongly suspect was supposed to be on Flight 93. On the day in August 2001 that al-Qahtani tried but failed to enter the U.S. at the airport in Orlando, Fla., Mohammed Atta, who would emerge as the leader of the hijackers, was waiting for him in the parking lot outside. "We have reason to believe," says a senior military official, "that Qahtani was supposed to be the 'muscle' on Flight 93."