Possible use of knives in hijackings
<Newsmax reports an ABC story that somehow a flight attendant on one of the doomed flights managed to communicate to the airline company that a knife fight took place while hijackers commandeered the airliner.
I have heard this from no other source so far, so take it with a grain of salt.
However, use of knives instead of guns may be a partial explanation for how four sets of hijackers got past four security posts.