Who 'police's' the police?

I live in a small town in Maine. A large number of the residents are unhappy with the police dept. They only enforce the law when it serves their purpose. They file incomplete accident reports, do not return calls from insurance investigators, do not know the posted speed limits in town areas and recently let a hit and run driver (a death resulted from the incident) without auto insurance off without any charges. Is there a state agengy that oversees local police departments?