Conspiracy theory rubbish, eyewitness reports as interviewed on the Discovery channel programme stated that the aircraft was last seen flying low, no mention of smoke was seen to 'wobble' rolled inverted and dived into the ground. Not much like a shootdown, which would be admitted anyway in the present circumstances.
To me that looks like a loss of control, considering the now well documented struggle onboard. Possibly it was a deliberate act on the part of the hijacker in an attempt to stop the passengers regaining control. There was a pilot among the passengers too.
I guess too that the hijackers were effectively lost, the most likely targets given the pattern of the was either the Whitehouse or the Capitol building, probably the Whitehouse. It fits the pattern of the day.
Most of us are pilots here, we know how difficult it would be for an inexperienced pilot to navigate visually in a high speed airliner to a point on the map off airways. Only the best military pilots are expected to do that. When you get there it is difficult to recognise a landmark. Witness how often pilots land at the wrong airport.
What all that tells me is the conspiracy theories are simply idle speculation by people who know no better or mischief making by people with an agenda.
Don't give them any credibility
Incidentally, I see from a newspaper reports that two Tornados were scrambled for a CAP over Sellafield after reports from Ireland to Scotland Yard and from a BMA staff member in Heathrow. True or False. I don't know.