Of course it was a small group that was directly responsible for this act of terrorism, otherwise, The NSA & CIA would have gotten wind of it, that's the nature of effective terrorism.
The pilots who flew those planes had to be damn good, they were trained somewhere, and they couldn't have gotten all those planes without inside help, unfortunately many of the key players were killed in the attack. There's a trail, and it will be followed, I'm putting my money on Afghanistan, the one true Moslem state that's not afraid of the U.S., (New York Post)