Federal Bureau of Investigation
Founded by J Edgar Hoover in 1924 and run by him till his death in 1972. Hoover amassed significant power abusing his authority. He used FBI resources to collect large amounts of compromising and potentially embarrassing information on many powerful people, especially politicians. Rumoured to have been gay, a cross dresser and being blackmailed by the Mafia. Just the sort you want running your secret police force.
Too jumped up and arrogant to refer to themselves as 'Police', prefer the term 'Agent' or indeed 'Special Agent'.
Formed to investigate US wide crime, and as a tonic to the hundreds of Law Enforcement agencies in the USA.