Among the white supremacist members of the Ku Klux Klan, Ron Stallworth stood out for a couple reasons: he was an undercover officer and he was a black man. In the fall of 1978, at the Colorado Springs Police Department, Stallworth saw an ad in the local newspaper calling for new members of the Klan. Intrigued, he sent off a letter using his real name, only expecting a brochure or pamphlet in response.

“I told him I hate ... anyone who isn’t pure Aryan white like I am,” Stallworth says, describing the contents of the letter.

About a week or two later, he received a phone call directly from Ken O’Dell, organizer of the Colorado Springs chapter of the KKK. That call would begin an astounding, seven month-long undercover operation that would take Stallworth down the rabbit hole of one of history’s most notorious hate groups.

The call came unexpectedly, however Stallworth played on what was already written in the letter. He even added to it, creating a sister who was dating a black man and saying “every time he puts his filthy black hands on her pure white body it makes me cringe.” That’s all it took for the O’Dell to take a liking to Stallworth’s character and request a meeting in person.

Of course, this represented a dilemma for Stallworth. When O’Dell asked how he’d be able to recognize him at their rendezvous point, he described a white undercover narcotics detective in his department with a similar build to him, a man Stallworth refers to as Chuck.

The fake, white Ron Stallworth would go out to meetings to collect intelligence while wearing a wire, while the actual Ron Stallworth would handle all calls and fill in his partner. And although Chuck and Stallworth had very dissimilar voices, the Klan never caught wind of the investigation.

As the undercover operation continued, Stallworth found himself in contact with David Duke, the leader and Grand Wizard of the KKK, over the phone. Their paths crossed when Stallworth called Duke to check on his membership application. Once Duke picked up, the initial 15-minute conversation became a weekly call between the two, with Duke unknowingly forming a bond with a black man.

During one ironic phone conversation, Duke told Stallworth how he could tell if the person he was talking to was black by their use of certain words. The word “are” in particular, Duke claimed, was a give-away. Duke told Stallworth that black people pronounce it as “are-rah” as opposed to “are.”

“And from that point on whenever I called him up, I would say ‘Hello Mr. Duke, how are-ah you doing?’” says Stallworth, amusedly. “I would basically use that to poke fun at the fact that he thought he was so self-righteously intelligent that a black man couldn’t be pulling a scam on him and he was being made a fool of the whole time.”

It wasn’t the only time Stallworth would fool Duke. As fate would have it, Stallworth had to work as Duke’s bodyguard in January 1979 when he came to Colorado to recruit new members. Despite objections to his police chief that the task could potentially ruin his investigation, the undercover Klansman was the only cop in the intelligence department available for the job.

When the two met face-to-face, Stallworth made it clear that he didn’t agree with Duke’s ideologies but would protect him as his duty required. Duke, surprisingly, was very cordial in their encounter, even shaking hands with Stallworth. The handshake Duke gave was the official Klan handshake—placing the index finger and middle finger against the wrist of the other person and wiggling the fingers as you shake. Most non-Klan members would probably not have even noticed that Duke used the special handshake—but Stallworth was aware.

Duke never recognized Stallworth’s voice, and before the pair parted ways they had their photo taken together. At the last second, Stallworth put his arm around Duke, causing the Klan leader to momentarily become flushed with anger.

After seven months, the investigation came to a close when O’Dell nominated Stallworth to become a chapter leader for being a loyal and dedicated Klansman. The development caused the chief to shut down the investigation before it could go any further. As a result of the operation, several Klan members were identified as Army recruits, though none were arrested.

The investigation as a whole was kept under wraps until an interview Stallworth did in 2006. He went on to publish a book, Black Klansman, detailing the investigation in full, and the book has now been made into a film, BlacKkKlansman, by Spike Lee.