''the flight went to a cruising altitude of 5.000 ft, their airspeed was around 300 kts at first, but then suddenly it went into a complete free fall it seemed, with the airspeed around 350 kts and crashed into the ocean''
so something happened in those last few minutes, it could be a problem with the tail section (which includes the horizontal and vertical stabilizer, cause if they had broken off mid-flight, it could explain the sudden loss of control)
this is my theory