Osama bin Laden revealed this on one of his tapes. From his perspective, the only way to really guarantee the success of the operation was to not even tell the agents involved what they were doing. When they got on those airplanes, some of them didn’t know they were on suicide missions.
Now, if the hijackers themselves didn’t know they were going into their own martyrdom to meet those seventy olive-eating virgins up there, how was anybody in the United States supposed to know? Yet we’re supposed to have the ability to figure it all out from a couple of non-specific FBI reports out of Phoenix?