"How did they get on board.." Perhaps you're not aware that the U.S. has a federal flight deck officer programme (FFDO) where selected and trained volunteer pilots are issued firearms in order to deter and defend against hijack attempts. Whatever your views onthe rights and wrongs, had an FFDO been on board on 9/11, that date would now be less memorable. Also, there have been numerous screw-ups by skymarshals and armed police; in one case a bar of plastic explosive being left on board. For some reason these don't attract the same publicity and crticism as a mistake by a pilot.