Having contributed to this one near the top, I walked to the PH and while en route I thought about UA93 a little longer and while my post above says basically 'I don't think it was shot down cos I don't see a need for a coverup', but I am a little confused by the smoke which was reported from the flight. If memory serves the gentleman on the phone also mentioned that the smoke was WHITE. To my mind that sounds not like a fire but CONDENSATION caused by sudden loss of cabin pressure. Hit by a missile or structural failure due to high G maneouvres? I don't know, but one of the two. The Gs would have to be incredibly high to break the cabin open, considering what other Boeings have flown through intact (TWA 727 and China AL 747SP mach 1 spiral dives, barrel rolls a party piece with the 707 til the LH training accident when they tried to do TWO). Unfortunately, as in TW800 (which I never thought was anything other than a mechanical failure), there is just enough unexplained here to give the conspiracy theorists a field day.
PS If someone let off a flare near the front, what would that look like to a passenger on the phone at the back of Y class?