I don't think UA93 was shot down for the simple reason that there were no "armed" aircraft around to do it.
I've seen interviews with a couple of guys from the DC guard who were preparing to fly a training mission that morning. They got scrambled with no missles and only "ball" training ammo. The emphasized that they were discussing the feasibility of ramming any "threat" aircraft since the likelyhood of shooting it down was remote.
I don't think anyone has pulled alert duty since the fall of the USSR and ANG airplanes simply aren't armed with real missles or ammo on a normal basis prior to 9/11.
Besides, what is the closest ANG unit to the 93 crash site? BUF, SYR, TOL, ACY?TC