"And I know that there was no single thing that might have prevented that attack."-Condoleezza Rice to the 9/11 Commission, 4/8/04
-- An intelligence report received in May 2001 indicating al Qaeda was attempting to send operatives to the United States through Canada to carry out an attack using explosives. This information had been passed on to intelligence and law enforcement agencies;-- Al Qaeda had been considering ways to hijack American planes to win the release of operatives who had been arrested in 1998 and 1999;
-- Osama bin Laden was set on striking the US as early as 1997 through early 2001;
-- Some intelligence suggested suspected al Qaeda operatives were traveling to and from the United States, were U.S. citizens, and may have had a support network in the U.S.;
-- At least 70 FBI investigations were underway in 2001 regarding possible al Qaeda cells/terrorist-related operations in the U.S.
-highlights from the Aug. 6th, 2001, PDB, via CNN.com
Maybe not one single thing, but all of these added up together could have. The PDB in question is in the process of being declassified, so we'll know what it says for sure. My thought is that the PDB won't amount to much, but it will sure underline the Bush administration being ill-prepared. A good summary of this viewpoint comes from Atrios.
August 6, 2001: Bush gets briefing titled "Bin Laden Determined to Strike Inside US."August 7, 2001: Bush begins month long vacation in Crawford, TX.
What else do you need to know?