And then if you consider the following picture from CNN that shows a piece of insulation falling off during take-off it starts to look like an accident that precipitated from that event.
For the shuttle to be brought down at that speed and altitude extremely advanced tech would have to be used, and the only nation that currently deploys such tech is Russia (they have an ABM defnse system around Moscow ....which is too far away for such an attack and anyways a launch would be easily picked up) and the US (and we have not yet perfected shooting down high-velocity objects). No terrorist group would be able to bring down the shuttle unless they messed with it before it went up.
As for the Challenger explosion in '86 that was due to a series of mistakes by NASA where they went ahead with a launch even when the freezing weather had caused severe problems with the launch vehicle! Heads rolled, and the blame rests solely on NASA for that particular case.
This one i think is an accident (after all once the shuttle went up there was nothing they could do about the fallen insulation).
I pray for the families.