The shuttle was so high and moving so fast, that we could not have shot it down ourselves, had we wanted to...stateless terrorists shooting it down is entirely impossible.
As for radio or computer tampering, that would leave fairly obvious evidence which would be found.
That leaves the option of putting a bomb on board or otherwise sabotaging the shuttle on the ground before takeoff, which is also extremely unlikely, but cannot be entirely ruled out before an investigation. One would think, though, that had there be sabotage, it would have destroyed the Shuttle on takeoff or in space, not on reentry.
Agreed. Why wait? Besides, it would be even more difficult (probably impossible) technically, which rules out sabotage IMO. Since the consensus is it can't be a missile, it has to be a malfunction.
Prayers for the crew and their families.