The FBI frequently investigated Martin Luther King, Jr. In the mid-1960s, King began publicly criticizing the Bureau for giving insufficient attention to the use of terrorism by white supremacists. Hoover responded by publicly calling King the most "notorious liar" in the United States. In his 1991 memoir, Washington Post journalist Carl Rowan asserted that the FBI had sent at least one anonymous letter to King encouraging him to commit suicide. Historian Taylor Branch documents an anonymous November 1964 "suicide package" sent by the Bureau that combined a letter to the civil rights leader telling him "You are done. There is only one way out for you..." with audio recordings of King's sexual indiscretions.

Did the FBI investigate Martin Luther King Jr.?