As I understand it, the police investigates police abuses. I find that very shocking. My own country has lots of the same issues, but at least it is a separate branch of police which investigates abuse, not a colleague who will have to deal with the backlash if they report dirty acts.
Also DAs have to prosecute these crimes. The same DAs who have to work day to day with the same police department.
There are huge conflicts of interest in there. We need both to end qualified immunity (I did not go into this, but it has soo many issues as well) and have an organization dedicated to investigate police abuse. It needs both to have the authority to do so and to be entirely independent from the accused.
note : in the context here this is not just police corruption (even though that's something that should be taken seriously) but any kind of police abuse.