Disagree.  I think there were passengers on the planes.  Too many eyewitnesses.  The planes were skyjacked remotely IMO.   Unknown on the Muslim angle.  Seems to me it would be easier to have the goatfuckers go to flight school, etc and build a back story and then disappear.  Planes get taken over, comms (cell calls, tower, etc.) can all be faked with state assets.