Engine Failure - Would have reported to ATC, it couldn't have been that catastrophic as to immediately down the aircraft.
Fire - Crew would have notified ATC.
Cabin Fire - Crew would have notified ATC.
Whatever it was it was abrupt and catastrophic, initial indications are the track bearing switched from 024 degrees to 333 degrees and altitude went from 35,000ft to 0ft along with the speed going from 468kts to 0kts in the space of one minute - As the flight was being monitored on secondary radar it would seem to me data supplied to the transponder ceased before the transponder eventually cut-out, and the aircraft for some as of yet unknown reason went down.
The only plausible theories I believe which led to the downing of the flight at this stage are;
Terrorism
Technical Nature - specifically in my thinking at the moment is an ADIRU failure previously experienced on a MAS flight - If this possibly happened on MH370, coupled with the fact the aircraft was over the Gulf of Thailand at 01:20 local Malaysian Time this could have had dire consequences if unexpected.
Catastrophic failure - Either an airframe component or it being caused by a sinister source.
What we are forgetting is that the crew were experienced, the captain was known for practicing on the flightsim on his days off and apparently helped in production with the PMDG 777X recently released for flightsim. And freakishly the same crew were filmed on 19 February flying from HKG to KUL on a 777-200 for a program on CNN's Business Traveller.
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