Generally speaking, the way it's supposed to work is the local prosecutors will start the process. That, unfortunately, isn't something they like to do because they have to work with police departments. If they fail to do their job, theoretically the next step is that the FBI gets involved. But, doesn't seem like today's FBI is doing much beyond prosecuting Trump's political enemies.

This is the reason why I've long believed we need a check both federal and local to police that is completely divorced from regular prosecution. We need lawyers/investigators whose sole purpose is investigating and prosecuting police at pretty much all levels of the government. The federal government theoretically has that with the office of inspectors general.