I've been reading the 9/11 comission report and it still doesn't seem clear as to what happened. The terrorist that was flying the plane seemed intent on putting the plane down since the second wave of passengers were overwhelming the 3 available hijackers. To top that off, the Terrorist pilot was maneuvering the plane as to knock the passengers off their feet. It also sounds like the plane was put down before a call to shoot the plane down was ordered.