My two cents:
Certain people in the CIA probably knew the attack was coming, perhaps even when and where. Yet they did nothing to stop it. Why? Because that's just the sort of crisis you need to get Americans to go to war.
America has been looking for excuses to kick some middle east ass for years now, this goes back to Bush senior and before him. Americans would be wise to never again vote in a president who has ties to oil.
This is probably no different than Pearl Harbor. It is likely that the Americans knew full well that Pearl Harbor would be attacked sooner rather than later, yet they "let" it happen. Why? To polarize public opinion and provide a prelude to war.