This Continental commuter broke up mid-air after it lost a tail because of a maintenance screw up. https://en.wikipedia.org/wiki/Continental_Express_Flight_2574 So mechanical failure is possible (although I personally lean toward a bomb based on the info provided). The electrical is a possibility but less likely.
I noticed CNN was floating ‘center fuel tank explosion’ yesterday. “ ...[I]t could have been a center fuel tank that might have exploded.” http://www.cnn.com/2015/11/02/africa/russian-plane-crash-egypt-sinai/index.html (quoting the former managing director of the NTSB)
https://en.wikipedia.org/wiki/American_Airlines_Flight_587
American Airlines Flight 587
The National Transportation Safety Board concluded that the cause of the crash was purely human error: overuse of the rudder mechanism leading to mechanical failure and not—as people had first feared—a type of terrorist attack.
https://www.youtube.com/watch?v=NpEXtFPlnKI
I just saw video of what looks to be a airliner exploding and coming down in a heavy smoke trail. Not likely that someone could be videoing from the ground just at the same time a bomb on board goes off on board. Makes more sense that they are pointing at the craft to record something they knew was coming.