According to BBC, there are unconfirmed (officially, that is) reports of smoke detection coming from, among other areas, the avionics bay. Fire is seriously bad news. Fire in the avionics bay ups the bad bit by a lot.
Pure speculations here: If there was a fire in the avionics bay, that could possibly explain why no mayday call was heard as monitoring systems, radio and transponders could be affected by this. A fire seems like a very likely candidate at this point but still, nothing is for sure yet. As for possible causes, that is a complete unknown at this point.
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