[Cicero]

I think it was probably a systems failure of some kind--notably the insulation and the tiles--or as this article suggests stress from the shuttle getting too far out of line with its flight path.

But you simply cannot rule out terrorism entirely without doing an investigation. There are numerous ways that the shuttle could conceivably be sabotaged, if a sleeper agent or a disgruntled fanatic had access to it, or possibly to a piece of equipment or one of the experiments that were carried on board. I'm sure everything was checked out carefully, but it would be hard to guard against every subtle form of sabotage. I think the odds are very much against it, but it's not impossible and it should certainly be investigated along with every other possible cause.

[VetoBill]

I remember once reading a book that dealt with a fictional destruction of a soviet shuttle.

A character in the book poured an amount of acid onto the ceramic tiles of a couple soviet shuttles. This supposedly roughened the tiles, allowing aerodynamic forces to compromise the shielding.

That being said I do not believe this was a terrorist attack. I do appreciate alternative theories on what could have happened, I just do not immediately associate them with terrorist activity. I believe most things that could have happen, most likely fall into the "stupid $hit that gets people killed but didn't mean it" category