Loss of oxygen is a common theory.
It kept in contact via ACARS for many hours.
Apparently its last requests indicate a possible systems issue.
Quote:
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The log-on message sent from the aircraft at 08:19:29 was "log-on request"; there are only a few reasons the SDU would transmit this request, such as a power interruption, software failure, loss of critical systems providing input to the SDU ....
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Given how long it had been in flight by then, running out of fuel is the most likely reason it eventually crashed.