We have a version of that in Ontario: The SIU, the Special Investigations Unit, which investigates every incident of police action that involves injury or worse in the province. It rarely finds anything wrong and can't really punish people. It's mostly staffed by former cops (because who else does investigative work? lawyers maybe? but would they work at their rates?)

There needs to be consequences for bad behaviour, but I think beyond that the solutions are further down the roots, in terms of the institutional culture that attracts and protects bullies, brutalizers.