In a Presidential Daily Brief (PDB) dated December 4th 1998, declassified by the CIA in 2004, where CIA source intel showed that the leader of Gamma Islamiyah, an Egyptian radical sect, and Osama Bin Laden, were planning to hijack aircraft and use them as weapons. This is more proof that the intelligence community and maybe even State Department going back to the Clinton administration, knew about the threat to airline security.