I can't help but suspect major pilot error/incapacitation here.
Incapacitation would have left the aircraft flying along at the last flight level and if LNAV engaged, then on the planned track. Any major event, such as decompression, that occurred to the aircraft would have been picked up by MAS Ops via ACARS.
Pilot error? After 2hrs plus and at FL350 the aircraft would have certainly been on autopilot. What error could they have made that resulted in the aircraft being lost?
Aircraft do not just suddenly fall out of the sky without some huge catastrophic event occurring. No radio calls at all were allegedly received (maritime operations monitor 121.5 too) and I am sure that slowly the events will be pieced together and a conclusion will be made.
It's extremely sad to read this morning's news and my thoughts are with everyone directly involved and affected by this incident.