According to currently available information, this particular case seems to be due to on an onboard explosion.

An anonymous spokesperson for SibAir claimed that the Tu-154 was off-course in flying over the Black Sea, so there is the possibility of some mechanical failure as well as terrorist intervention.

The plane is said to have gone down in an area of the sea which is quite deep, so recovering the black boxes may be a problem. It is possible that some ground station may have recorded communications with the plane, but these are not likely to be available in the first hours after the crash.

Tu-154's have been in service since the early 70's and is a workhouse of the various descendents of Aeroflot. There have been several spectacular crashes of this model, but as far as I know, there is no instance of a Tu-154 just exploding due to mechanical failure.

A bomb is a possible cause for this crash.

If it was a terrorist action, it seems to me to be a stupid one, since such actions are sure to further unite the Americans and the Russians - not a good omen for the terrorists.

sp