Origins of the September 11 attacks
The attacks were influenced by the Bojinka plot, a terrorist operation planned by Khalid Sheikh Mohammed and his nephew Ramzi Yousef, who was responsible for the 1993 World Trade Center bombing.[5] The plan would have included bombings of eleven trans-Pacific airliners and crashing a plane into the CIA Headquarters. Yousef tested the plan by planting a bomb aboard Philippine Airlines Flight 434 on December 11, 1994, which detonated but only killed one passenger. The plot was intercepted when Yousef's Manila apartment burned down and the Philippine National Police captured his laptop computer with the plans. Yousef himself was captured by U.S. and Pakistani forces in 1995.[11]
Khalid Sheikh Mohammed presented a modified plan to bin Laden in 1996 in Afghanistan.[5][12] According to the 9/11 Commission, Khalid Sheikh Mohammed envisioned hijacking twelve airplanes on both the East and West coasts, and for eleven of them to crash into the World Trade Center and the Empire State Building in New York City; The Pentagon in Arlington, Virginia; the Prudential Tower in Boston, Massachusetts; the White House and the United States Capitol in Washington, D.C.; the Willis Tower (then Sears Tower) in Chicago, Illinois; the U.S. Bank Tower (then Library Tower) in Los Angeles, California; the Transamerica Pyramid in San Francisco, California; and the Columbia Center in Seattle, Washington.[13][14] Nothing came of the idea at the time, however, as bin Laden rejected the plan as being too elaborate.[5][9]
In December 1998, the Director of Central Intelligence Counterterrorist Center reported to President Bill Clinton that al-Qaeda was preparing for attacks in the U.S., including training personnel to hijack aircraft.[15]