Such an organization is called "gendarmerie" and it is seen in many countries like France, Italy, Spain, Argentina,... usually alongside the civilian police force.
This seems like a simple case of local prosecutors and juries not doing their work. The officers were acquitted in the Rodney King trial too. (And that's why currently people are bending backwards trying to argue against Quantified Immunity - in civil suits! These cases should never even reach that level. These should be criminal cases, and the police/city/county/state/country should automatically compensate the wrongly harmed.)
The federal level should step in and charge/indict officers. There might be obvious constitutional issues though. So the next best thing could be an oversight system that is actively against bias (racial, ethnic, and occupational in case of somehow the local system consistently siding with local police). Of course it's unlikely that such a law could survive for long and still be effective, so this probably some other solution is needed - maybe/probably something like tying federal funds to implementing better state-level oversight, etc.
Organizations like the FBI, the Department of Homeland Security, the National Guard, and the Bureau of Alcohol Tobacco and Firearms collectively fill a similar role to the gendarmeries in other countries. I'm not sure we need another armed federal agency to enforce the law.
Such an organization is called "gendarmerie" and it is seen in many countries like France, Italy, Spain, Argentina,... usually alongside the civilian police force.