One of the people apparently shown in the hundreds of hours of videotapes destroyed by the CIA showing CIA operatives torturing people was Abu Zubaydah. He named three Saudi princes (2 in their 40's and 1 in his 20's) and a Pakistani Air Force General as being directly involved in 9/11. Abu Zubaydah apparently gave the private phone number and cell phone of one of the top Saudi princes. This doesn't necessarily mean that the tape mentioned the four. All four have mysteriously died afterwards, and the CIA claims to have destroyed tape. If they did, it was as part of its cover up operation. They don't want to produce it to any outside investigators. It is too incriminating of the powers that be. The CIA had previously denied access to two tapes to the 9/11 Commission and to some defendant's attorneys. They didn't divulge the existence of the tapes during the discovery period. They lied about having turned over everything they had.
That's the way it always is around a major cover-up. People die. If one studies the President Kennedy assassination, one finds many people dying usually sudden deaths in a cluster of time that defies statistics.
Those four (3 Saudis and 1 Pakistani) died, because they knew too much about U.S. involvement and their names were leaked by Abu Zubaydah. They killed them so they couldn't talk.