According to Field McConnell, all of the planes used on 9/11 had a feature known as Boeing Honeywell Uninterruptible Autopilot. The Boeing Uninterruptible Autopilot is a system designed to take control of a commercial aircraft away from the pilot or flight crew in the event of a hijacking.
Furthermore, all of the passengers were flown to Diego Garcia, an abandoned naval base at the time, where all died.