The FAA received piles of reports in 2001 warning of Al Qaeda plane hijackings in the United States, but laughed them off. More bizarre, however, is the fact that the Bush administration blocked the release of those documents until just recently -- and even now, they're heavily redacted. The NY Times:
The Bush administration has blocked the public release of the full, classified version of the report for more than five months, officials said, much to the frustration of former commission members who say it provides a critical understanding of the failures of the civil aviation system. The administration provided both the classified report and a declassified, 120-page version to the National Archives two weeks ago and, even with heavy redactions in some areas, the declassified version provides the firmest evidence to date about the warnings that aviation officials received concerning the threat of an attack on airliners and the failure to take steps to deter it.
Full article here.