Their background should be checked, their houses searched, the flight simulator carefully scrutinised etc etc - in the hope that some clue to this mystery, however small, may be turned up.
Agree, however the posts here using the simulator as proof of foul play are a joke, as it's been said people have hobbies, private planes, radio controlled, all sorts just because it's different to others doesn't make it evidence.
If he had a SCUBA qualification and crashed in the water then he must be guilty, imagine if he did skydiving as a hobby.