The 9/11 Commission Report: The Planes Operation

According to Khalid Sheikh Muhammad, he began thinking of ways to attack the United States after his nephew, Ramzi Yousef, returned to Pakistan following the 1993 WTC Bombing. Like Yousef, Khalid Sheikh Muhammad reasoned he could best influence US policy by targeting the country's economy. He would propose the idea of hijacking ten airplanes and having them crash into various US targets to Osama Bin Laden in 1998 where it was put on hold for Al Qaeda leadership to mull over. By spring 1999, Bin Laden approved of the operation but selected the pilot hijackers himself.

The 9/11 Commission Report: Chapter Five: Part 5.2 The Planes Operation