Former FBI Director Louis Freeh, My FBI, pg. 289:
But in theory – and I stress theory – if we had been able to do that, and if we had connected that information with the arrest the next month in Minnesota of Zacarias Moussaoui, the French-Moroccan who aroused a flight instructor’s suspicions when he asked to learn how to fly a commercial airliner, and then tied that to the two al Qaeda cell members living in San Diego and to the earlier warnings that terrorists were plotting to use commercial flights as kamikaze planes…then perhaps 9/11 never would have happened, or would have happened at a lesser scale.
Washington Post, 3/21/06:
An FBI agent who interrogated Zacarias Moussaoui before Sept. 11, 2001, warned his supervisors more than 70 times that Moussaoui was a terrorist and spelled out his suspicions that the al-Qaeda operative was plotting to hijack an airplane, according to federal court testimony yesterday.