There is evidence that several corrupt DoJ, DoD and CIA employees had taken bribes from Al Quida in exchange for information and logistics support during the four years prior to 9/11.

When one considers that the 9/11 terrorists could take jumbo jet flying lessons, move about the US unobstructed, live for several weeks less than 2 miles from NSA head quarters and be on State Department watch lists, even without the supporting evidence of bribery this makes sense.
If FBI and DEA where less concerned with enriching themselves by stealing from suspects and taking bribes and more concerened with doing their jobs, 911 may never have happened.