The U.S. Federal Bureau of Investigation (FBI) began monitoring Martin Luther King, Jr., in December 1955, during his involvement with the Montgomery bus boycott, and engaged in covert operations against him throughout the 1960s.FBI director J. Edgar Hoover was personally hostile toward King, believing that the civil rights leader was influenced by Communists.
https://kinginstitute.stanford.edu/encyclopedia/federal-bureau-investigation-fbi