A male cabin crew was arrested a few years ago at a UK based airline I worked for. Had been viewing Al-Qaeda material on the 'net and was picked up by MI6.
Big strong guy, looked like a real gym rat, brought us coffee in the cruise a few times, no harm was done but you've got to wonder. If he had a well executed plan then he could well bring the plane down even with two pilots in the flight deck. Where does it end?