Has there been any discussion regarding the background checks of the flight attendants? Certainly they would be somewhat familiar with the cockpit layout from sitting in there while the pilots go on bathroom breaks. If one of them had ill intentions and a modicum of flight-simulator experience they could easily have murdered the remaining pilot (using the axe or any other heavy object) while the other was outside, and initiate a sequence of events leading to this aircraft's disappearance. An FA with a long-stewing intention of accomplishing something like this could gain all the information they would need to shut off various systems (communication, dump cabin pressure, etc) simply by asking questions while seated in the cockpit over various flights. Not to mention studying any number of websites and messing around with X-Plane or whatever.
Let's face it it's not rocket science here. Pull a couple of circuit breakers, put on an O2 mask, manipulate some cabin pressure controls to dump the cabin, turn the altitude knob and pull, type a few keystrokes into the FMS, and that's it. A little homework is all it would take.
If according to the latest (CNN so take it with a grain of salt) all passengers and the pilots have been cleared of suspicion yet they are STILL viewing the aircraft's maneuvers as a "criminal act", then that basically just leaves the cabin crew, correct?