I stand to be corrected here but in the UK at least I was always of the opinion that the Civil Authorities had to be called in to investigate:
Murder
Manslaughter
Treason
Treason Felony
Rape
This has always been and no date always will be even if this is a suspicious death at a Top Secret Radar Station...
It is no good waiting a few weeks to decide if an offence may or may not have been committed, The forensic evidence needs to be preserved right from the 'get go!'
A crashed aircraft may or may not fall into that category and until the facts are established then is it not better to err on the side of caution? ...