American aviation officials were warned as early as 1998 that al Qaeda could "seek to hijack a commercial jet and slam it into a US landmark," according to previously secret portions of a report prepared last year by the September 11 commission. The officials also realized months before the September 11 attacks that two of the three airports used in the hijackings had suffered repeated security lapses.

http://www.truthout.org/docs_2005/091405L.shtml