According to Sky News a few moments ago, everybody's worst fears realised, ie. Initial heli overflight of the crash site has revealed no survivors
.
What might lead us to suspect an inflight catastrophic event would be the fact that there was no mayday call in well over 10 mins of (fairly) rapid descent. Further, there appears not to have been any deviation in heading which seems pretty inconceivable had the crew been conscious and trying to fly the aircraft.
Grim indeed...