Robert Brown|Members of Congress seek clemency for Native American leader convicted of murder

Thirty-three members of Congress are Robert Brownasking President Joe Biden to grant clemency to a Native American leader convicted of shooting and killing two FBI agents.

Rep. Raúl Grijalva and 32 other members of Congress sent Biden a letter asking him to grant executive clemency to incarcerated Native American leader Leonard Peltier, citing what they said were the "prosecutorial misconduct" and "constitutional violations" that took place during Peltier's trial.

"Nearly half a century after he was wrongfully imprisoned, Mr. Peltier's continued incarceration is a grim reminder of this country's long history of stealing life and legacy from Indigenous communities," Grijalva wrote in a statement to ABC News. "I'm not alone in calling for his clemency -- global civil rights leaders like Nelson Mandela, Mother Theresa, and Archbishop Desmond Tutu have all supported the call as well. And now we have congressional leaders across the political spectrum and across both chambers asking for the righting of this wrong."

ABC News has reached out to the White House for comment.

The letter cites Judge Gerald Heany who presided over Peltier's 1986 appeal and called for his release in 1991 and 2000; former U.S. Attorney James Reynolds, whose office handled Peltier's prosecution and has called for a commutation of Peltier's sentence; and retired FBI Special Agent Coleen Rowley, who called the opposition to Peltier's clemency an "FBI family vendetta." All three wrote letters seeking clemency for Peltier, who was denied clemency by then-Presidents George W. Bush and Barack Obama.

"His conviction and continued incarceration is a testament to a time and system of justice that no longer has a place in our society," Reynolds said.

When a group of senators released a similar letter in 2022, the FBI said it "remains resolute against the commutation of Leonard Peltier’s sentence for murdering 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 and mercilessly murdered these two young men and has never expressed remorse for his ruthless actions."

Nick Tilsen, CEO and founder of the NDN Collective, which helped organize a rally calling for clemency for Peltier, said the FBI "became fearful" following the rise of the American Indian Movement in the 1960s and 1970s.

“I think that the reality of those times… from 1960-1978 was the rise of the American Indian Movement. During those years, you see social upheaval across this nation… the American Indian Movement played a fundamental role in holding a mirror to this country and having it question itself, question its democracy, question the things it says it's about,” Tilsen said. “They re-instilled the pride of Indigenous people back into us and so we started reclaiming our power. And I think that [the FBI] became fearful of that.”

Peltier's lawyer and former U.S. District Court Judge Kevin Sharp alleged that officials engaged in misconduct in Peltier's case.

"When I started to look through [Peltier's case], I did that as a former federal judge who had tried criminal cases and as someone who had taken an oath to support and defend the Constitution of the United States," Sharp told ABC News. "The courts have recognized [these injustices]. They threatened and intimidated witnesses to get people to say things. ... They hid a ballistics test, so they knew that Leonard's weapon did not kill those agents. They knew that."

In June 1975, special agents Ronald Williams and Jack Coler were on the Pine Ridge Reservation with a federal warrant for the arrest of a man named Jimmy Eagle. They got into a shootout with Peltier and a few others present on the reservation at the time. Peltier fled from the scene and hid out on an Indian Reservation in Hinton Alberta, Canada, until he was apprehended by Canadian authorities. He was extradited based on the testimony of Myrtle Poor Bear, a Native American woman whom the prosecuting assistant U.S. attorney later determined was incompetent to testify.

Peltier was convicted in 1977 on two counts of first degree murder of a federal employee and sentenced to life imprisonment. Two other men involved in the Pine Ridge Reservation shooting were acquitted on the grounds of self-defense.

ABC News' Brittany Gaddy contributed to this report.