The researcher who discovered a full unredacted version of a vicious letter from a J. Edgar Hoover deputy to Martin Luther King, Jr. in 1964 called it the “Holy Grail” of documents.

The letter, which accused King of extramarital affairs and urged him to end his own life, was available in censored form for decades, but the unredacted version reveals more details. Yale historian Beverly Gage came across the document when going through photos she had of “super-special secret files” from Hoover’s office that the National Archives had possession of, according to NPR.

She said while she knew there could be new material in there, she had no idea what she actually had. “I was amazed,” she said. “This really is one of the most famous documents that there is. It’s sort of like the Holy Grail.”

In all likelihood, Hoover didn’t write the letter himself, but had Assistant Director of the FBI William Sullivan do it for him. Sullivan sent the letter anonymously, pretending to be a disgruntled civil rights worker. The letter suggested there was an accompanying audio recording of King’s affairs, and called the civil rights leader “evil” numerous times. The letter arrived at King’s residence just days after Hoover called him the “most notorious liar in the country” after King said the FBI was derelict in its duties in protecting black people in the South.

What Gage found most interesting about the letter is that it was actually a last-ditch effort to get rid of King: the FBI had shopped the information to media outlets, but the press wasn’t interested. At the time, the media was generally not willing to write about affairs of prominent figures, avoiding the topic with rumors about John F. Kennedy as well. They were also well aware that this was a bald-faced effort to discredit King, and the civil rights movement itself.

Gage said the letter is a good example of why the United States needs to keep a tight reign on intelligence agencies to keep them from obtaining information that can give them power of people’s lives. She noted that the inquiry into King’s personal life began as a national security issue as officials looked into ties between him and communists. The investigation quickly delved into his personal life, however.