Has this been proposed at all?
Is there anything weird about american policing/law that would mean a national, federal agency wouldn't be able to take on this role?
Sometimes police are found to have violated federal civil-rights law, but that is a bit of a sketchy backdoor.
The Right Thing would be for each state to create such a commission, and for the Congress to create one for federal law-enforcement agencies.
Most Constitutional rights extend to people against the states. So if your civil rights (due process, etc) have been violated, it's against federal law.
Ferguson just agreed to one: http://www.npr.org/sections/thetwo-way/2016/01/27/464610005/...
- Serpico