The new movie “BlacKkKlansman” tells the story of a Colorado Springs police effort in 1978 and 1979 to infiltrate the Ku Klux Klan, improbably led by a black detective named Ron Stallworth.

The Spike Lee-directed film focuses on the investigation, but Stallworth is haunted by thoughts of what might have happened had the investigation been allowed to continue longer than six months.

Stallworth’s investigation never resulted in arrests but produced evidence of a Klan resurgence in Colorado Springs, including a presence at nearby military bases. It also put police officers near men who would become leaders of a burgeoning white nationalist movement.

Stallworth began the investigation by responding to a Klan recruiting ad placed in the Colorado Springs Gazette-Telegraph’s classified section in October 1978. After the Klan made contact with Stallworth by mail and phone, then-Colorado Springs Police Chief John Tagert authorized detectives to conduct an undercover investigation.

Tagert, now 85 and living in Cascade, says he recalls the Klan investigation but doesn’t remember the details four decades later. “I thought very highly of Ron (Stallworth, then a young detective). He did excellent work for us, I do remember that.”

Stallworth continued phone contacts with Klan leadership in Colorado Springs, Denver and in Louisiana, where he reached Grand Dragon David Duke. A white officer, identified in Stallworth’s book as “Chuck” and in the movie as “Flip Zimmerman,” was used for face-to-face meetings with Klansmen. In those meetings, the white officer used Stallworth’s name.

“During the course of this investigation, we had made contact with the Posse Comitatus, which was big in Colorado Springs. The American Nazi Party out of Denver. The Klan chapter out of Denver. And the grand dragon from Alabama, Don Black, had made an appearance in Denver, had come down to Colorado Springs to meet with our guys in one of the meetings I’d set up,” Stallworth said in an interview in El Paso, Texas, where he grew up and now lives. “We had this confluence of white supremacists from Denver and Colorado Springs in one setting. My thinking, when I was ordered to shut down the investigation, was these were the people that we could possibly have penetrated even further.”

The Colorado Springs police penetration of the Klan was so convincing that the local leader, a Fort Carson soldier who was leaving the Army and returning to Texas, began promoting Stallworth as his successor. According to Stallworth, Tagert ordered the investigation shut down in April 1979 over concerns about entrapment if an undercover officer rose to leadership ranks in the Klan.

Tagert said concerns about entrapment always arose in undercover investigations. Stallworth said he disagreed with the decision but understood Tagert’s reasoning.

“Had we been successful and continued the investigation, perhaps we could have placed somebody in the Nazi Party and in all the other right-wing organizations in Denver, and basically done a similar thing – penetrated them, gain intelligence. If they cross over from intelligence into the criminal, then make the arrest. But at least we could have really understood what we were dealing with in the state of Colorado. That opportunity was lost,” Stallworth said.

In November 1979, seven months after the Colorado Springs police investigation ended, one of the main targets of the investigation – Denver Klan leader Fred Wilkins – walked into a Denver radio station, waved a gun at talk show host Alan Berg and threatened to kill him. Charges against Wilkins later were dropped after he reached a settlement with Berg, according to a 2017 dissertation by Michael Adam Lee, a doctoral student at the University of Colorado at Boulder.

In the early 1980s, Posse Comitatus and other “Christian identity” groups began to take shape as a more virulent form of white supremacy, in Colorado and elsewhere. One group to emerge around 1983 was an Idaho-based organization called The Order. In June 1984, The Order assassinated Berg, the Denver radio talk show host who was an outspoken critic of white supremacists.

“When I heard about the Alan Berg killing, that’s probably the first thing I thought of. Had we been able to pursue this, we might have known about the plot to kill him before it happened and could have prevented it,” said Stallworth, who was working in Wyoming law enforcement at the time of the Berg murder.

The getaway driver in the Berg hit was David Lane, who attended at least two meetings that were part of Stallworth’s Klan investigation. He left the Klan soon after that investigation and eventually connected with The Order, also known as the Silent Brotherhood, in 1983. He died in prison in 2007 but continued writing hate tracts while jailed and is still an icon of right-wing extremist groups. His “14 Words” remains a mantra for white supremacists.

Lane’s 14 words – “We must secure the existence of our people and a future for white children” – are emblematic of a significant change from the white supremacy movement of the civil rights era. During the civil rights era, white supremacists were defending their longtime power structure in the South and elsewhere; by the 1990s, white supremacy began to fight what it called “white genocide,” said Mark Pitcavage, the senior research fellow with the Anti-Defamation League’s Center on Extremism.

“David Lane ended up having a tremendous stature in the white supremacist world, revered as a hero and a martyr,” Pitcavage said.

Even if the Colorado Springs investigation had continued, Pitcavage said it was unlikely to have derailed Lane, who didn’t take a violent turn until several years later. He also believes Tagert made the right decision to shut down the investigation when it became likely that undercover officers might be asked to take a leadership role among white supremacists.

“With undercover investigations, whether it’s organized crime or extremist groups, one of the cardinal rules is your undercover agent or informant cannot be in a position of leadership or influence,” Pitcavage said.

In “BlacKkKlansman,” director Lee ties the Klan of Stallworth’s investigation to the re-emergence of white nationalists exemplified by Charlottesville.

Stallworth left the Colorado Springs Police Department in 1980, concerned that two supervisors who didn’t like him would be an obstacle to his career. He first moved to Arizona, then Wyoming, then finally in 1987 to the Utah Department of Public Safety, where he became a gang expert. He retired from law enforcement in 2011 and wrote the book “Black Klansman” two years later.

The movie is a fictionalized rendering of Stallworth’s book. The screenwriters created a racist cop who is not in the book and shifted the timeframe to roughly 1972, to make the Vietnam War part of the backdrop.

Even in retirement in El Paso, Stallworth still thinks of the Berg killing and what might have been a chance to prevent it.

“It still crosses my mind. I still think about it. We had an opportunity, and we didn’t follow up. And you only get those opportunities once in a million times. That was our millionth time, and we didn’t take it,” Stallworth said.