MrDuck - could be lots of things. Has the onboard oxygen supply been used? Are there fire-damaged panels? Evidence of a struggle in the cockpit, or that people have tried to force the cockpit door? If the plane was uncontrollable but the passengers were still alive, we might find video on phones, last letters, etc.
The CVR and FDR may well contain meaningful data too; for example they should cover whether or not there were control inputs during the descent (which could be the difference between everyone being dead, the pilot(s) deliberately crashing, and the pilot(s) doing their best to achieve a water landing).
If/when they do find it, I suspect that they'll be able to get a pretty good idea of what happened.