A quote from Frank Gardner - the BBC's security correspondent:-
There were no distress calls that we know of, which would imply that something sudden and catastrophic took place that gave the crew no time to put out a distress call. This is 2016 - planes don't just fall out of the sky. The suspicion is that it's an act of terrorism of some form, which means was it shot down or was a bomb put on board, if that is the case.
I trust his words more than most!