Various reports state (and I can't swear to the accuracy, but I think I've seen it enough that it is confirmed) that the gearbox was changed on January 17th and the rotorhead on March 27th - 2 days before the accident.
I believe that apart from a couple of short maintenance flights, this was the first full passenger flight since the rotorhead change. I would imagine that will be the focus of the investigation right now.
The one witness statement that made me think possible gearbox failure / seizure was a comment that the in addition to the strange noise and a bang, the helicopter was seen to rotate in the air before an explosion, but this could be just about anything and may have been post rotor detachment.
Either way, witnesses on the ground seem to have been drawn to look up, film, look for the aircraft due to a strange noise, at least several seconds before the crash, so it suggests (in some ways similar to BND85N) that there was a short period of intense noise and gearbox stress before breakup.
Terrible tragedy. If this is an engineering / maintenance issue (and that is an 'if', the aircraft could be signed back to fly fairly quickly. Whether the passengers would accept it is another matter?