IT was one of the most extraordinary police undercover operations in US history - a black cop infiltrating the Ku Klux Klan.

The incredible story of Ron Stallworth is told in Blackkklansman, Spike Lee's new movie that's released in the UK next week.

Stallworth writes about his time at the Colorado Springs Intelligence Unit in 1978 in his book, Black Klansman: Race, Hate And The Undercover Investigation Of A Lifetime.

It all began when the Ku Klux Klan placed an ad in a local newspaper.

Stallworth wrote in for more information and received a call two weeks later from a man named Ken O’Dell, who introduced himself as the organiser of the Colorado Springs KKK.

After convincing O’Dell with a rant about “n*****s, Jews and Mexicans", the man revealed plans for four cross-burnings.

Stallworth writes: "My pen paused over my notes as I heard this. Four burnings here in Colorado Springs? Terrorism, plain and simple.”

O’Dell concluded the call by saying he’d like them to meet the following week, and asked the officer how he’d recognise him.

“How would I, a black cop, go undercover with white supremacists?” Stallworth writes.

“I immediately thought of Chuck, an undercover narcotics cop I work with who was about my height and build.”

He gave Chuck’s description to O’Dell and arranged their first meet.

When Chuck was enthusiastic about the job, it was decided that Stallworth would play himself on the phone and Chuck would play him in person.

Chuck would wear a wire in the field so Stallworth could listen in and Stallworth would debrief Chuck on all of his calls.

Fast-foward to Stallworth picking up a KKK pamphlet with an ad suggesting readers call in to hear “The Voice of the Klan.”

Stallworth called the number and was greeted by a recorded message of hate.

Suddenly, the message was cut off mid-thought by a human voice.

“Hello?”

“Hello,” said Stallworth. “Who is this?”

“This is David Duke, the actual voice of the Klan.”

They spoke for 15 minutes. Stallworth heaped praise on Duke and his organisation, and Duke said he hoped they could meet.

Soon, Stallworth and grand wizard Duke were speaking once or twice a week.

Duke told him about rallies planned in cities including Kansas City and Los Angeles, and Stallworth passed the information right along to the police, preventing several rallies that were cancelled due to heavy police presence.

In time, he was able to learn the proposed locations of the Colorado Springs cross-burnings and those areas were flooded with police as well.

Soon he learned that Duke himself was scheduled to visit Colorado Springs on January 10, 1979, hoping to attract recruits and generate new enthusiasm for the Klan’s message.

The morning of Duke’s visit, Stallworth was summoned by his chief, who informed him that, despite his ongoing investigation, Stallworth would be protecting Duke.

Protecting dignitaries was one part of Stallworth’s regular duties, and several other undercover officers, working with Stallworth, were already planted in the Klan.

He was the only officer available.

Stallworth wasn't going to let this opportunity go to waste.

Careful not to reveal his name, and only introducing himself as a police officer, he showed his guts by playing a prank on the grand wizard.

After asking for a photo, he waited until the last moment and threw an arm around Duke, making the pair appear friends.

Duke attempted to grab the Polaroid, but Stallworth was faster.

Unfortunately the picture has since been lost.

Two months later, at the end of March, Stallworth was blindsided by an order to shut down the investigation.

No reason was given, but Stallworth had his suspicions.

Over the course of the operation, three undercover officers had been officially sworn into the Klan as part of the investigation.

Stallworth believed the chief feared a PR nightmare if word got out.

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While his investigation produced no arrests, it did prevent rallies, uncover valuable intelligence and dislodge Klan members from key national security positions.

Most importantly to Stallworth, though, was preventing the cross burnings in Colorado Springs.

“As a result of our combined effort, no parent of a black or other minority child had to explain why an 18-foot cross was seen burning,” he writes.