Part of the MO of the 911 terrorists was to announce they had a bomb, but apparently they only had a fake bomb belt. (No explosives to trip any sniffer sensors or cause any sniffer dog to become suspicious).
Just repeating what's already been said... think about it. You have an international layover for customs and boarding passengers. At this time the pilot cabin is open and personnel are moving about possibly even in the jetway...how easy to hijack it from there...