Indigenous activist Leonard Peltier will return home nearly five decades after he was imprisoned for the 1975 killings of two FBI agents.

US President Joe Biden commuted Mr Peltier's sentence Monday following decades of community-led advocacy, the Associated Press reports.

The White House said Mr Peltier, who is now 80 and in poor health, will transition to home confinement. The commutation is not a pardon for crimes committed, a decision welcomed by some of Mr Peltier's advocates, because he has always maintained his innocence.

The National Congress of American Indians celebrated the "historic" decision in a statement saying the case "has long symbolised the systemic injustices faced by Indigenous Peoples".

Former FBI Director Christopher Wray had objected to the release in a private letter sent to President Biden earlier this month and obtained by The Associated Press. Mr Wray claimed Mr Peltier "is a remorseless killer".

However, many human rights and legal experts have observed that the case against Mr Peltier was weak, and his conviction a miscarriage of justice.

Mr Peltier was active in the American Indian Movement, which confronted police brutality and discrimination against Native Americans since the 1960s.

Tashina Banks Rama, who is Oglala Lakota and Ojibwe of the Leech Lake Band and the daughter of AIM co-founder Dennis Banks, told National Indigenous Times AIM began in Minneapolis in reaction to police brutality and other injustices.

"The American Indian Movement was a movement of men and women who were fighting to be recognised. They were fighting against police brutality that was taking place in at the time Minneapolis. The beginning of AIM was 1968… the environment of Minneapolis was really harsh for Native people," she said.

"The Native people in Minneapolis were being arrested by the busloads, you know, a police paddy wagon would pull up to the back of back of a bar, and at the front of the bar the police would come in… and so people would start leaving out the back and the paddy wagon would be right there, and they would just load people up.

"Police brutality was at its worst in the late '60s against Native people. And that was when people like my father Dennis Banks, Clyde Bellecourt, Vernon Bellecourt, and other leaders came together and formed the American Indian Movement. It started out as a community-organised group that were trying to protect each other, and look out for one another. And then it just grew very quickly from there."

In 1973 the movement reclaimed the village of Wounded Knee on Pine Ridge — the Oglala Lakota Nation's reservation in South Dakota — leading to a 71-day standoff with federal agents.

Mr Peltier has said he was present and involved in the fight during the June 26, 1975, FBI raid.

Agents Jack Coler and Ronald Williams were shot in the head at close range, the FBI said.

AIM member Joseph Stuntz also was killed.

Mr Peltier escaped to Canada but was later extradited to the United States and convicted on two counts of first-degree murder.

He was sentenced to life in prison in 1977, despite defence claims of falsified evidence.

Two other movement members and Mr Peltier's co-defendants, Robert Robideau and Dino Butler, were acquitted in the killings.

Mr Peltier was denied parole as recently as July, and wasn't eligible for parole again until 2026.

KVRR reports that as word was starting to get around about their brother's upcoming release from prison, Sheila Peltier was on the phone with her sister in Fargo, North Dakota.

"Never been so happy in my whole life you know because my brothers, my big brothers, coming home," Betty Ann Peltier Solano told KVRR.

"The prosecutor said you couldn't even prove that the bullet even came from Leonard, you know , Leonard's gun," she said.

"You could see that it was one-sided. Leonard was innocent, he's innocent. And for and after all these years nothing can give back the years that the government took from him."

Mr Peltier helped establish scholarships, a newspaper, and a gift drive for the Native American youth and students during his time in prison.

"He's our warrior, he's our great warrior, he stood up for us. People are going to want to be around him and just meet him. It's going to be a lot for him to take in. I hope he can handle it. I mean he's a strong man, but after fifty years of being by yourself it's going to take some getting used to," said Sheila Peltier.

The North Dakota Collective, an Indigenous-led organisation, is working with the family to find out exactly when Mr Peltier will be released.

"Also because of him, he made the world a better place for the Native Americans. Just sorry it had to take so many years from him for that to happen," said Sheila Peltier.

The Associated Press reports that Chauncey Peltier, who was 10 when his father was locked up, said Monday he was shocked and thrilled.

"It means my dad finally gets to go home," Mr Peltier said.

"One of the biggest rights violation cases in history and one of the longest-held political prisoners in the United States. And he gets to go home finally. Man, I can't explain how I feel."

Mr Peltier's tribe, the Turtle Mountain Band of Chippewa, has a home ready for him on the Turtle Mountain Indian Reservation in Belcourt, North Dakota, his son said.

Mr Peltier's lawyer said his release date was tentatively set for February 18.

Amnesty International has long considered him a political prisoner. Advocates for his release included Archbishop Desmond Tutu, civil rights icon Coretta Scott King, actor and director Robert Redford, and musicians Pete Seeger, Harry Belafonte and Jackson Browne.