Leonard Peltier, an indigenous activist who executed two wounded FBI agents after a shootout in 1975, is due for what could be his final parole hearing Monday.

And while he has the support of left-wing advocates and a handful of Democratic lawmakers, the FBI Agents Association is vehemently opposed to his release from prison and calling on the federal Parole Commission to keep him locked up.

Peltier, 79, is being held at a federal penitentiary in Sumterville, Florida. He is serving two consecutive life sentences for the slayings, plus another seven years for an attempted escape.

On June 26, 1975, FBI special agents Ronald Williams and Jack Coler were looking for a group of armed robbery suspects in the Oglala Sioux Indian Reservation in Pine Ridge, South Dakota. Although Peltier wasn't one of them, he was traveling in a vehicle that caught the agents' attention.

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According to court documents, Williams warned Coler over the radio that someone in the vehicle was about to start shooting at them.

Gunfire erupted. Both agents were wounded.

Peltier approached them with a rifle and shot each man in the head from point-blank range. Then he fled to Canada, where he was captured and extradited to the U.S. to face justice.

Coler, originally from Bakersfield, California, had been an LAPD officer before joining the FBI in 1971. Williams was also a California native, from Glendale. He joined the FBI in 1972.

Four men were arrested in their deaths, but only Peltier was convicted, according to the FBI. The government dropped charges against James Eagle, the robbery suspect Williams and Coler were looking for when Peltier killed them. Two other men, Robert Robideau and Darrelle Butler, were acquitted at trial in 1976. 

Peltier's supporters have argued that his initial 1977 conviction was based on shoddy evidence and "prosecutorial misconduct." But he failed to have it overturned after more than a dozen appeals, including two that reached the Supreme Court.

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"Just because it's been several decades doesn't mean that the impact of his activity has lessened," said Natalie Bara, the president of the FBI Agents Association, a professional group that represents 14,000 active or retired special agents.

Speaking on her own behalf, and not for the FBI, she said he deserves to remain behind bars. 

"His actions on that day have had lasting consequences for the FBI family, for the two agents' families, and I think if you were to release him from prison, it sends a terrible message to not only rank-and-file agents who are out on the street every day risking their lives for the American people, but it sends a terrible message to people who would consider doing harm to an FBI agent."

FBI Director Christopher Wray said in a statement that "the FBI remains resolute in our opposition to Leonard Peltier’s latest application for parole. Peltier was convicted of the brutal murder of FBI Special Agents Jack Coler and Ronald Williams at South Dakota's Pine Ridge Indian Reservation in 1975. We must never forget or put aside that Peltier intentionally murdered these two young men and has never expressed remorse for his ruthless actions.

"Over the past 45 years, no fewer than 22 federal judges have evaluated the evidence and considered Peltier’s legal arguments, with each reaching the same conclusion:  Peltier’s claims are meritless, and his convictions and sentence must stand. In addition, Peltier’s crimes include a post-conviction escape from federal custody, during which he and his crew fired shots at prison employees. Granting parole for Peltier would only serve to diminish the brutality of his crime and further the suffering of the surviving families of agents Coler and Williams, as well as the larger FBI family."

A group of senators, including Elizabeth Warren, Bernie Sanders and Mazie Hirono, sent a letter to Attorney General Merrick Garland in March asking him to grant Peltier a "compassionate release."

They did not include the names of agents Williams and Coler in their letter.

Brian O'Hare, a former president of the FBI Agents Association, sent his own letter to Garland urging him to deny the clemency request.

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"The effort to lionize the murderer of Special Agents Coler and Williams remains an ongoing source of pain for the FBI family," he wrote. "Special Agents Coler and Williams made the ultimate sacrifice, and that sacrifice should be honored — Peltier should serve the full sentence for his heinous crimes."

Even after his conviction, Peltier continued to embrace violence against law enforcement, O'Hare added.

He took part in a 1978 prison break from Lompoc Penitentiary in California, during which the escapees fired at prison guards. Peltier had another seven years added to his sentence.

"Peltier has been far from a model prisoner, and would never be considered a candidate for clemency but for his status as a political celebrity," O'Hare wrote.

Peliter was an activist with the American Indian Movement, or AIM, at the time of the shooting. He later claimed to be a political prisoner.

Peltier's last parole hearing ended in a rejection in 2009. Although his supporters have the backing of some prominent Democrats, including President Biden's secretary of the interior, Deb Haaland, former presidents Bill Clinton and Barack Obama both denied requests for clemency.

Although Peltier has his supporters, many within the FBI family adamantly oppose his release, Bara said.

Ed Mireles, the first FBI agent to receive the bureau's Medal of Valor after a deadly gun battle with two suspected killers in 1986, scoffed at the idea Peltier should receive a compassionate release.

"[Peltier] executed the two agents when they were wounded on the ground – no mercy or forgiveness," he told Fox News Digital. "No mercy or forgiveness for Peltier."

Fox News' Emmett Jones and The Associated Press contributed to this report.