The state of New South Wales, Australia, has an independent "Law Enforcement Conduct Commission" whose explicit job is to investigate law enforcement misconduct (across all major state law enforcement agencies), and to refer cases to prosecutors. Some other Australian states have similar agencies. There is also a federal equivalent, the "Australian Commission for Law Enforcement Integrity", although it does not appear to have been as effective as its state-based equivalents in uncovering law enforcement misconduct.
Maybe the US could copy the idea?