(Reuters) - Native American activist Leonard Peltier was denied parole on Tuesday after serving nearly five decades in federal prison in the 1975 killing of two FBI agents, a conviction that his supporters say was tainted by prosecutorial misconduct.

Peltier, a member of the militant American Indian Movement in the 1970s, was part of a group of Native American men who engaged in a shootout with FBI Special Agents Jack Coler and Ronald Williams on the Pine Ridge Indian Reservation in South Dakota in June 1975.

The agents, who went to the reservation in search of a fugitive, were killed, along with one of the Native American activists. Peltier, 79, has maintained that he was not the one who shot Coler and Williams.

In June, Peltier had his first parole hearing since 2009 at a federal prison in central Florida. FBI Director Christopher Wray, in a June 7 letter to the top federal parole official, called Peltier a "remorseless killer" who should never be freed.

Peltier's supporters say that prosecutors withheld critical evidence that would have been favorable to Peltier at trial and fabricated affidavits that painted him as guilty.

Peltier's lawyer plans to appeal the U.S. Parole Commission's decision. Advocates, among them the late Nelson Mandela, a former prosecutor and judge involved in the case, have long called for his freedom.

"This decision is a missed opportunity for the United States to finally recognize the misconduct of the FBI and send a message to Indian Country regarding the impacts of the federal

government's actions and policies of the 1970s," Kevin Sharp, Peltier's lawyer and a former federal judge, said in a statement.

Wray praised the decision, saying in a statement that justice prevailed again. "No amount of prison time will ever change the facts surrounding the murders of FBI Special Agents Coler and Williams," he said.

The Parole Commission did not immediately respond to a Reuters request for comment.

In 1976, two other men were found not guilty in the deaths of the FBI agents on self-defense grounds.

Peltier fled to Canada before the trial. He was eventually extradited back to the U.S. and tried separately in 1977. He was found guilty and given two life sentences.

Peltier has an interim hearing about his parole status scheduled for 2026 and a full hearing scheduled for 2039, NBC News reported, citing Sharp as a source.

(Reporting by Tyler Clifford; Editing by Cynthia Osterman)