At this point it was either hijacked, with crew involvement or something catastrophic happened to the oxygen supply system, with on board foreign agent involvement, then the plane flew itself out of fuel while the crew and passengers passed out dead due to lack of oxygen.
If it was hijacked it was probably flown into the sea after the transponder was shut off. A classic muslim jihadist/ pilot tactic that has happened at least three times in the recent past.

Now time to wildly speculate…………
Worst case and least likely scenario is Iranian agents aboard gassed the cabin, killing everyone within a couple minutes. Then the jet was flown remotely into Brunei or Indonesia, then on to Iran and is being refitted into a flying cruise missile as we speak. Destination probably Israel or less likely the US, with anything from bio weapons to nuclear weapons on board.
A one in a million plot that would require multiple state and corporate sponsors, along with months of training. They would need unfettered access to a $300 million state of the art commercial jet for training (not a Cessna) and there’s only a few nations on earth where you could pull that off without anyone noticing, Iran, China, Russia and Saudi Arabia being a few.
Iran cannot reach Israel with nuclear weapons unless they use jets and they’re not going to use a jet that can be traced back to Iran. So the only option is to steal one.
Extremely unlikely plot, but nowadays nothing surprises me anymore. Not to mention someone stole a 727 just a few years ago. That jet has yet to be found either……….Jihadists always say “you might have the watch, but we got the time”……….Now two jets are missing and there’s a fatwa to detonate two nukes on US soil that’s been in effect now for over a decade. Last I checked there’s never been a fatwa that hasn’t been carried out. Either way, time will tell. At this point hopefully it was just another jihad pilot nose diving into the sea…….It’s almost criminal to keep letting jihadists fly commercial airliners……Wonder when we will finally wake up!