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Surely such allegation ought to be investigated by an independent group not by another force. Such a group could have the weight of law behind it and be a 'commission' to independently investigate complaints against the police. It could be called the Independent Police Complaints Commission or IPCC for short. Then if it investigates, there's less concern about the thoroughness and independence of any investigation. Of course if a body like the IPCC existed and the police used other police to investigate allegation against them it might raise a flag about impartiality.