Edit: lots of people saying fbi. You all are putting a lot of faith that an FBI investigation will stop the cops before they kill you or someone you love during a “routine” traffic stop.

The FBI actively covers for corrupt cops.

Graham Dyer was tazed to death. The cops' story is that he was beating his head against the walls, so they had to subdue and restrain him. But his parents received the orginal, uncensored video and audio, and said that one of the cops could be heard saying "I'm going to kill you." (They had a motive to kill him, too: he bit one of them.)

The FBI took the footage and muted it.

The feds don't care about police brutality. They just do whatever they think will appease people. If the case is low-profile, they'll cover it up before too many people notice. If the case is too high-profile to cover up, they'll investigate it to give the appearance of "fighting police brutality" and appease people.