As PukinDog suggests, other rules apply as soon as it appears a crime may have been committed; the French authorities asked the German police to investigate the first officer's parental home and his private flat, and to interview his associates; delay may cause evidence to vanish...
The AAIB in the UK always releases a prompt preliminary report on an air accident if action can be taken to avoid repetition.
Anger and blame often felt by loved ones, even after a medical mistake. How difficult for those who have had to wait months and years for government agencies to find answers. It would be wrong to hide reasonable information about the cause.
The Americans and now the Australians now have a rule that two people must be in the cockpit at all times. A trusted and experienced cabin crew member present would make it more difficult for a rogue pilot to carry out his plan undisturbed and unobserved.