Remora
This story is going to come out in pieces, and the alert reader will have to do the cut and paste. The intelligence failure that allowed the successful hijacking of four planes has a backstory, but we haven't heard it yet. So like some jigsaw puzzle, we will have to look around for the odd news item to piece it together. This article.
FBI Knew Terrorists Were Using Flight Schools (washingtonpost.com) in the WP is extremely noteworthy. Graf two:
"Three days after the attack on the Pentagon and the World Trade Center, FBI Director Robert S. Mueller III described reports that several of the hijackers had received flight training in the United States as "news, quite obviously," adding, "If we had understood that to be the case, we would have -- perhaps one could have averted this."

Graf three:
"A senior government official yesterday acknowledged law enforcement officials were aware that fewer than a dozen people with links to bin Laden had attended U.S. flight schools. However, the official said there was no information to indicate the flight students had been planning suicide hijacking attacks."

The last sentence has more than the whiff of deniability. Oh, so there might have been indications that they were planning plain vanilla hijackings?