That's what you're supposed to assume. But the problem is, a man dead, or at least missing. Sure, police is not always a clean organisation, and police in USA even less so. They've been known to cover up murders in the past. But first of all, those are usually within the unofficial "code of conduct" among the officers, and secondly, even then there's a procedure to be followed.

Defenders aren't police officers, they are outside the "buddy conduct", there's a dead man that people will be asking about, and there's just literally no reason not to investigate what happened.