They were able to measure the latency and compression of the inmar sat handshakes with the plane to determine more precisely where the plane was when it transmitted its last handshake. This was either a bowling gone terribly wrong, or I would imagine there was an onboard fire which knocked out the electrical systems. Smoked could have suffocated everyone on board, thus you had a zombie plane flying into it ran out of gas. In any case a horrible tragedy.
It doesn't need to be a fire (because I think if it was the case there would have been a alarm signal launched which was not the case), if the air filtering/ pressurizing system partially fail, pilots due to lack of oxygen could misdjudge everything as the brain does not work properly in this condition and they'll pass out eventually, another possibility is a total electrical faillure but anyway, it will takes months if not more to find the black boxes
face the truth we don't even know our own planet properly yet. 
