I don't think it's an act of terror. At least not by an organization. It's a single individual with intimate knowledge of the plane who is able to shut down several communication systems and pilot the aircraft for several hours. My guess is one of the flight crew; if that person incapacitated the rest of the flight crew or lured them out of the cockpit, the new hardened cockpit doors would make it almost impossible for the rest of the crew or passengers to gain access to the cockpit.