The FAA warned the airports in early 2001 that if 'the intent of the hijacker is not to exchange hostages for prisoners -Yahoo! News

Yahoo! News - US aviation authorities received pre-9/11 warning on al-Qaeda plans: report:

"So many reports on Osama bin Laden (news - web sites) or his al-Qaeda terrorist network were received by the FAA in the months before 9/11, that it conducted classified briefings for security officials in 19 of the busiest airports in the United States.

The report said the 52 intelligence reports sent to the FAA in the months leading up to the September 11 attacks, comprised half of all the intelligence summaries it received.

The FAA warned the airports in early 2001 that if 'the intent of the hijacker is not to exchange hostages for prisoners, but to commit suicide in a spectacular explosion, a domestic hijacking would probably be preferable.'"