[bootless]

There's no way they could shoot it down from the ground or elsewhere. I speculated that it was an internal altitude-sensitive explosive...

[Ichneumon]

I speculated that it was an internal altitude-sensitive explosive...

Speculations of sabotage are extremely premature at this point. Re-entry is a risky enough procedure all by itself that it could easily cause such a disaster due to any number of simple mechanical failures. In fact, it's amazing that the US space program has gone as long as it has without a re-entry failure -- the Russians have had several, for example.