Either a missle or an on board detonation by the cabin....a suicide bomber with a small amount of C-4 could avoid metal detection, rush the cockpit area and detonate....
This would cause enough damage to destroy the cockpit and/or incapacitate the pilots at the critical moment of approach....(the new reinforced doors are not bomb proof)
C-4 is undetectable by metal scanners and a small basting cap would slip through also.....Batteries could be used from a carry-on radio, cell phone etc.
Whatever happened had to happen fast because none of the passengers or crew would tolerate any shenanigans......
NeverGore