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.
The surveillance failed to show that King was a communist, but it did result in many tapes of extramarital sexual liaisons by King. So the next year, Sullivan sent the following unsigned letter to King's home. An unredacted version of it was only recently unearthed by Yale historian Beverly Gage, and published in the New York Times in November 2014: