When the Rev. Dr. Martin Luther King Jr. delivered his “I Have a Dream” speech before huge crowds on the National Mall in August 1963, the FBI took notice.

“We must mark him now, if we have not done so before, as the most dangerous Negro of the future in this Nation from the standpoint of communism, the Negro and national security,” FBI Domestic Intelligence Chief William Sullivan wrote in a memo two days later.

A massive surveillance operation on King was quickly approved, and FBI Director J. Edgar Hoover became increasingly fixated on proving that King had communist ties and discrediting him generally.

According to Enemies, journalist Tim Weiner's history of the FBI, Sullivan "had a package of the King sex tapes prepared by the FBI's lab technicians," and sent it along with this letter to King's home. The package was opened by King's wife.