As Ron Stallworth prepares for the nationwide release of BlacKkKlansman, a film based on his experience as a police officer infiltrating the U.S.’ most notorious white supremacy group, he has repeatedly fielded an obvious question: How could a black cop pull something like this off—even by phone—with people whose identities hinge on whiteness?

Stallworth says it is partially because of his teen years in El Paso, where he was one of only a handful of black students for much of his education. El Paso, which is historically less than 5 percent black, is where Stallworth returned after retiring from more than forty years in law enforcement. “All my friends were white. I learned at an early age how to traverse the white world, the white-dominant world. I learned and I was successful at it. I learned the nuances, I learned how to act, how to be, but I always was conscious and aware of my blackness.”

The movie, released August 10, is set in Colorado Springs, where Stallworth led a police intelligence investigation that penetrated the Ku Klux Klan all the way up to David Duke, then the grand wizard of the organization. Produced by Jordan Peele, directed by Spike Lee, and starring John David Washington as Stallworth, BlacKkKlansman uses the 1979 investigation to take a broader look at the repeated resurgence of white supremacy in America over the past four decades.

Ronnie, as Stallworth was called through his school years, first moved to El Paso at four years old, when his mother decided to move from Chicago in 1957 to live closer to her sister. The family first settled on Yandell Street, in the heart of a historically black central El Paso. Stallworth went to school at Alta Vista Elementary, which had one of El Paso’s largest African-American populations following the 1955 desegregation of the city’s schools. “I went to school with a lot of kids whose fathers and mothers were part of the El Paso black history. You know, first achievers in some area or had gone to school at Douglass (the city’s historically black school) and so forth. Of course, at the time I didn’t know the significance of that, they were just kids next door that I played with,” he said.

In the mid-1960s, as I-10 cut through the traditionally black neighborhood, El Paso’s African-American population began to scatter from its historically segregated base. When he was in sixth grade, Stallworth’s mother moved him and his younger brother to northeast El Paso, where he attended Burnett Elementary School. He said he and his brother were the only two black children at the school. “Everything else is lily white or Mexican,” he said. “For me at that age, it was a culture shock. I learned to fit in mainly because I was an athlete.”

For his seventh- and eighth-grade years, Stallworth went to Bassett Junior High School, where he recalled being one of only five black kids in the student body. The other black students were from military families assigned to nearby Fort Bliss, and Stallworth didn’t feel any kinship with them. Most of his friends were white. When he entered the ninth grade in 1967, Austin High School had a fairly large black population, including many of the black children he knew from elementary school. “I had a dilemma that I had to confront. Was I going to hang out with the white kids that I had gone to school with the past three years from Burnett to Bassett? Or was I going to re-associate with my black friends that I grew up with initially at Alta Vista?” Stallworth said.

“I chose to confront it by hanging more with my black friends from Alta Vista days, but not giving up the white friends from the other two schools,” he said. “So I would sit at the table with the black kids during lunch, and we’d do our banter back and forth. But occasionally I’d get up and I’d go sit down with the white kids and chat with them and what not. Of course, because I come from the black table they would look at me like, ‘Why are you here?” he said.

After graduating in 1971, Stallworth moved with his mother to Colorado Springs, where her sister then lived. Ron joined her in part because the city’s police department offered a cadet program for aspiring police officers ages seventeen to nineteen. His job interview was conducted by two white police officers and a black city personnel manager. Colorado Springs at the time had no black officers. “So the two white guys are asking these racial questions,” Stallworth recalled. “I think I heard [the n-word] three or four times during the course of my interview.” Only the white police officers used that language, he said.

Stallworth was hired as a cadet and eventually promoted to police officer. He became the first black detective in the history of the Colorado Springs Police Department. In 1979, he saw a classified ad in a Colorado Springs newspaper recruiting Klan members and responded to it out of curiosity. After the head of the Klan in Colorado Springs called him, Stallworth’s superiors agreed to start the undercover investigation monitoring Klan activity in the area, which police weren’t previously aware of. In the operation, which was re-created in BlacKkKlansman, Stallworth conducted phone conversations with Duke and other Klan leaders, while a white detective was sent to any face-to-face meetings.

He left the Colorado Springs police force in 1980, a year after the investigation ended. Stallworth worked at law enforcement agencies in Arizona and Wyoming before landing at the Utah Department of Public Safety in 1987, where he became an expert on gang issues. In 2013, he wrote a book about his Colorado Springs undercover exploits called Black Klansman.

In 2010, Stallworth reconnected with an Austin High classmate named Patsy Terrazas. Both had been widowed. Patsy said she had a crush on Ron in tenth grade, something he says he was not aware of at the time. They quickly became close while planning their fortieth high school reunion in 2011, and were married in 2017. The two live in El Paso, just a two-minute walk from their old high school. The film has thrown them into the spotlight, attending red-carpet premieres in Brooklyn and Los Angeles and speaking at Alamo Drafthouse events in El Paso, Austin, and Denver. “We both enjoy the perks, the benefits of it. But we’re also aware of the challenges in terms of staying out of the fray and not getting caught up in the hype,” says Stallworth. “It’s like I tell people: Patsy and I are just two El Paso kids who good fortune has fallen upon and we’re going to enjoy this good fortune for as long as we can.”