UA93 was shot down

Overview
There is a theory that UA93, the plane which crashed in Pennsylvania on 9/11, was shot down by the US military – contradicting the official story that the passengers, having learned of the other airplane-based attacks via cellphone, took over the command cabin and crashed the plane rather than allowing it to be used against a target.

Evidence thus far points to the idea that the plane did crash near Shanksville (contradicting the extreme "Flight 93 landed safely" theories), that the passengers did attempt a take-over, but that the plane nonetheless crashed because it was shot down rather than due to the actions of those on board. If true, the action of shooting it down would seem to be the right and proper action to have taken under the circumstances, and the action of concealing this truth from the public could turn out be relatively forgivable (though nonetheless reprehensible) – assuming it does not turn out to have been for any reasons beyond "making heroes" out of the passengers (which they already were, even if unsuccessful in their takeover).

Support

 * Evidence Indicates Flight 93 Was Shot Down: "The evidence shows that Flight 93 did indeed crash near Shanksville, and suggests that the passengers did struggle to gain control of the plane. However, the cause of the crash was apparently trauma to the aircraft – such as a missile strike – rather than the actions of whoever was in the cockpit."
 * 2008-04-11 "The US Air Force Shot Down Flight 93": an anonymous poster relays second-hand testimony strongly indicating that the Air Force shot down UA93. The details given should be checkable at least for realism, although this wouldn't prove that it actually happened (someone with recent military experience could presumably make up a story with convincing details). If true, it gives some indication of the social pressure any potential non-conspirator witnesses may be under to keep quiet: "anything that went against the Official, media delivered story was viewed as unpatriotic. I knew that I loved this country, so I kept my mouth shut."

Need Facts

 * Links to recordings of cellphone conversations (which at least one site claims have not been made available)
 * Eyewitness claims of an apparent missile strike on UA93
 * Any other arguments used to support this theory (here's a collection: )

Counterarguments

 * This would indicate an arguably proper response to the situation, i.e. something along the lines of the military scrambling jets to intercept, the aircraft not responding to hails, and the military deciding – given circumstances in DC and NYC – to shoot down a hijacked civilian airliner over sparsely inhabited territory rather than risk its use as yet a third missile. If so, why hide the true story?
 * There is a Vanity Fair article which sheds considerable light on what probably happened, without directly addressing the issue of why the physical evidence suggests a shoot-down. (Possibly that resemblance is because of the difference between this crash and all prior crashes: whoever was in charge was trying to crash, and hence may have aimed the plane more or less straight down.)
 * It seems clear that airline passengers are (now) certainly willing to risk their personal safety (and perhaps that of the airplane, though there have as yet been no further incidents where crashing the plane was the only way to prevent its use as a missile) to foil terrorist plots. Prior to 9/11, most airplane-based terrorism was best handled by negotiation, and passengers were best advised to go along calmly and quietly in order to prevent unnecessary deaths. In the wake of 9/11, where it is well known that the stakes can consist of much more than the airplane and its occupants, people have begun fighting back:
 * 2002-02-16 The Shoe Bomber's World
 * 2003-05-29 Stabbing would-be hijacker subdued
 * 2006-12-26 Passengers thwart Russian plane hijacking

why deny the shootdown?
Possible motives for hiding the shootdown (and claiming instead that passengers took over the cockpit and deliberately crashed the plane):
 * Fear of criticism for the shoot-down order, regardless of how completely justified it probably was
 * Admission of a shoot-down would open the door to inquiries -- who shot the plane down, on whose authority? Where are the in-flight recordings from the interceptor planes doing the shooting? Where are the recordings of the dialogue between planes and ground control? If the plane was crashed by the passengers, it can be claimed that the military was not involved; no names need to be mentioned, and there are no transcripts to ask for.
 * A false cover story which is very difficult to refute helps distract overworked and unpaid unofficial investigators (the official investigators being safely under control) and leaves more questions unanswered, providing rich soil for disinformation to propagate.

Inconclusive Evidence

 * Multiple eyewitness accounts describe the plane "going straight down" without any mention of outside interference, but others heard loud bangs (Note: could "bangs" have been caused by structural failure at high speed due to improper handling of aircraft controls?)