In Wisconsin the law is that if police are involved in a shooting, the incident has to be investigated by another police department in a different jurisdiction. It stems from a case in Kenosha where a young man was handcuffed and shot in the temple by a cop. It was ruled a “justified” shooting by Kenosha police. They (the family)did some research and found that all of the police shootings in the country (over 400) were ruled “justified”. So, they made this law and I don’t think it goes far enough. They all belong to same union.
I seem to recall years ago some big city (NY?) had a Civilian Review Board to investigate police wrongdoing. Might be time to revisit this idea.