Speculation, speculation!!!
I doubt fuel contamination. If engines stop, aircraft will glide and radio communication can be established and chances are good that aircraft don't explode with engines stalling.
An eye witness need not be a rocket scientist to confrim an aircraft exploding before hitting the ground.
Something catasrophic happened in the cruise with an extreme high rate of descent (possibly uncontrolled flight) it sounds like with no chance of radio communication. Possibility exits that the crew were not contious at the time of impact due the the fact nothing was said on the radio.
Must wait and see!