The U.S. Parole Commission on Tuesday denied parole for Leonard Peltier, a Native American activist serving a life sentence for his conviction in the 1975 slayings of two FBI agents, despite an array of calls for clemency over the years from such luminaries as Pope Francis and the Dalai Lama, as well as tribes, civil rights groups and federal lawmakers.

Peltier won't be eligible for another parole hearing until June 2026, the commission said.

Peltier, 79, had faced a parole hearing June 10, where his attorney, Kevin Sharp, argued that the enrolled member of North Dakota's Turtle Mountain Band of Chippewa Indians was wrongly convicted and that his health was failing.

Sharp could not immediately be reached for comment Tuesday.

Calls for Peltier's release have echoed for decades among Native American rights groups. However, the pressure on federal officials to grant the activist clemency have amped up in the last several years.

Peltier contracted COVID-19 in 2022 and is suffering from several chronic health conditions, including a potentially fatal abdominal aortic aneurysm, according to Amnesty International.

Peltier's case has received widespread and growing support from both faith and human rights leaders — including the pope, Saint Mother Teresa, Nelson Mandela, the Dalai Lama and Coretta Scott King, according to the federal lawmakers who urged U.S. Attorney General Merrick Garland in a March letter for a compassionate release.

U.S. Sens. Brian Schatz and Mazie K. Hirono of Hawaii, Elizabeth Warren and Edward J. Markey of Massachusetts, Tina Smith of Minnesota and Peter Welch of Vermont, all Democrats, penned the letter, in addition to Bernie Sanders, an Independent of Vermont.

In addition, those previously involved in his prosecution, including James H. Reynolds, the U.S. attorney who oversaw the prosecution and appeal of Peltier's case, stated that the "conviction and continued incarceration is a testament to a time and a system of justice that no longer has a place in our society," the lawmakers said.

Peltier was convicted in 1976 in the killings of FBI agents Jack Coler and Ronald Williams on the Pine Ridge Reservation in South Dakota. According to the FBI, Peltier and his supporters claimed that they had traveled to the reservation from Farmington, New Mexico, with a contingent of American Indian Movement members.

The AIM members, according to various Native American activist groups, had gone to the reservation to help quell violence among the political opponents of tribal Chairman Richard Wilson.

Wilson, the groups said, created the Guardians of the Oglala Nation — a private militia that was said to have attacked his political opponents.

Despite evidence of prosecutorial misconduct and due process violations that mounted throughout Peltier's trial, according to the lawmakers' letter, he continues to be detained.

Late U.S. Circuit Court Judge Gerald Heaney, who presided over Peltier's 1986 appeal in the Eighth Circuit, also publicly called for his release — first in 1991 and again in 2000 — detailing the injustice of the trial and proclaiming that "a healing process must begin," according to the lawmakers.

According to Amnesty International, a key witness in the case who is said to have been harassed and threatened for months by the FBI into testifying against Peltier later recanted her testimony. However, she was not allowed to be called as a defense witness at his trial, the nonprofit said.

"Today is a sad day for Indigenous Peoples and justice everywhere. The U.S. Parole Commission's denial of parole for Leonard Peltier, America's longest serving Indigenous political prisoner, is a travesty," said Nick Tilsen, president and CEO of activist group NDN Collective, in a statement Tuesday. "They denied parole to a survivor of genocidal Indian boarding schools as he struggles to survive this unjust incarceration, they insist on holding him for a crime for which they have no physical evidence against him. Clearly, the parole commission — which is supposed to be an independent body — was influenced by the FBI. The FBI continues to abuse its power, promote false narratives, and engage in counterintelligence activities. The FBI has no regard for the Constitution or the laws they have sworn an oath to protect."

The FBI had disputed Peltier's claims of innocence.

In a statement Tuesday, FBI Director Christopher Wray said that "justice continues to prevail" and that Peltier "has been afforded his rights and due process time and again, and repeatedly, the weight of the evidence has supported his conviction and his life sentence."

The recent call for Peltier's release isn't the first time federal lawmakers have urged clemency for the Native American activist.

Thirty-three members of Congress in October 2023 called upon the Biden administration to grant Peltier's clemency, saying key figures involved in his prosecution have stepped forward during the course of his incarceration that underscored the constitutional violations and prosecutorial misconduct that took place during the investigation and trial that led to his conviction.

"Now, more than ever, bedrock principles of justice warrant your consideration of a grant of executive clemency or support of compassionate release at the Federal Bureau of Prisons," the members told the administration.

--Editing by Michael Watanabe.