Investigate "accident"? Or "crime"?
Observation, just above:
"... I doubt that this crash was suicide. ... don't get upset ... investigation will look into the matter...."
In decades past, sometimes, a core-group of "accident" investigators embraced a rumored scenario -- "pilot suicide", sabotage, the "Boeing Scenario", the missile scenario.
Investigator-err does happen: the investigators NEVER study investigator-err. [But they want you to study pilot-err.]
The main idea for investigators should be to focus on the direct evidence (trail of debris, examination of wreckage, recorded data, observations of witness-crew); discard or suspect manufacturer's created-scenario, "analysis", simulator trials. Accident investigators are not "crime" investigators.