However, what happens if the AAIB during the course of an investigation discover that something has been done to the aircraft which was deliberate and therefore put the aircraft/pilot and public into danger (like cutting the brake line on a car, causing the brakes to fail). Do they then have to report the matter to the police and provide the evidence they have found in their investigations,
I can't quote chapter and verse, but my understanding is that if AAIB consider that a crime has been committed, it ceases to be an accident investigation and becomes a criminal investigation which they hand over leadership to the police.
The classic example of that is Lockerby. AAIB quickly identified that a crime had been committed - a bomb on board. They then handed the investigation *leadership* over to the Scottish police, whilst continuing to be actively involved throughout in an expert supporting role.
G