Air traffic control once seeing the planes are off course and not willing to change course are supposed to alert NORAD (I think it stands for North American Aerospace something Response Air Defense?) and then a decision is made to scramble military jets to possibly escort and possibly shoot the plane down if a danger to the public at large is determined. From what I remember air traffic control did not alert NORAD fast enough with the flights that were off course, they failed to follow protocol. I don’t know if once notified if NORAD also was slow to respond? I have no idea exactly was done by NORAD. Even if the jets were escorting the planes that wound up crashing, there are all sorts of diffultiess to consider. Being over a major city there are obviously dangers in shooting down the plane. Also, if they had downed the plane, there would be all sorts of second guessing about whether it was necessary and the US government would be under scrutiny. Since 9/11 had many flights crash, there would probably have been only a reasonable amount of second guessing, but imagine if our government had shot down all the planes that crashed before they crashed into the buildings, Twin Towers, Pentagon, or even in PA. Imagine the consipiracy theories and backlash.