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:

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.

"There is only one thing left for you to do. You know what it is."

"You have been on the record — all your adulterous acts, your sexual orgies extending far into the past. This one is but a tiny sample," the letter says. It threatens that the public "will know you for what you are — an evil, abnormal beast." It continues: "Your 'honorary' degrees, your Nobel prize (what a grim farce) and other awards will not save you. King, I repeat you are done."

Though the letter was unsigned and the letter writer appeared to want King to think he was black — "you know you are a complete fraud and a great liability to all of us Negroes," one line states — King and his advisers quickly concluded the letter had been sent by the FBI, Gage writes.

"There is only one thing left for you to do. You know what it is. You have just 34 days," the letter says, apparently urging King to commit suicide. Fortunately, King didn't back down. But the letter is a terrifying reminder of what government surveillance agencies can be capable of.

(Hat tip: Catherine Thompson of TPM)