Yes this is a speculation but I believe we need to look into crew suicide. Only this time, the crew member didn't want anyone to find out it was suicide and so he/she proceeded to take the plane stealthily to a spot no one would be able to find it.
I'm sure the suicide plan took into account that previous suicides by pilots were discovered by investigators to be suicides due to blackboxes.
I sure hope though that the plane landed/bellylanded somewhere with the passengers still alive...