The United States has a highly decentralized and fragmented system of law enforcement, with around 18,000 law enforcement agencies, and is regulated separately in the 50 states, plus US territories, the federal and local level as well. Hiring and disciplinary standards vary greatly between police departments, the majority of which are small in size. There is no national database of dismissed officers, who may or may not be de-certified to practice law enforcement by their jurisdictional regulatory agency, often called POST (Peace Officer Standards and Training) agencies, or by a similar agency. Some states have established a state-level database or taken other administrative measures to try to prevent dismissed officers from being rehired, but these databases are not centralized, nor do they have uniform prerequisites. The large population and land area of the country makes it further harder to notice these cases; an officer may have moved hundreds of miles away to join another agency and the new agency would be unlikely to have learned of the case in the media.
