Washington DC - A group of United States senators has once again called for the release of Indigenous freedom fighter Leonard Peltier, the country's longest-held political prisoner.

"Mr. Peltier, who has been imprisoned for the past 49 years and is suffering from severe health conditions, should be able to return home and live out his remaining days among his own people," a group of seven Democratic senators wrote in a Friday letter to Attorney General Merrick Garland.

The action was led by Brian Schatz of Hawaii, chairman of the Senate Committee on Indian Affairs. Fellow signatories included Mazie Hirono of Hawaii, Ed Markey of Massachusetts, Bernie Sanders of Vermont, Tina Smith of Minnesota, Elizabeth Warren of Massachusetts, and Peter Welch of Vermont.

"It is time that the federal government rectifies the grave injustice of Mr. Peltier’s continued imprisonment, and strongly urge you to allow for his compassionate release," the senators said.

Last October, a bipartisan group of US lawmakers signed a similar letter calling for Peltier's release just ahead of Indigenous Peoples' Day amid strong public pressure from Indigenous activists and allies.

The demand for Peltier's freedom has also gained support from the United Nations Working Group on Arbitrary Detention.

A member of the Turtle Mountain Band of Chippewa, Peltier is currently incarcerated in a maximum-security facility in Coleman, Florida.

The American Indian Movement activist was taken into US custody after he was convicted of killing two FBI agents in a shootout on the Pine Ridge Indian Reservation in South Dakota in June 1975.

But numerous irregularities in the 1977 murder trial have called into question Peltier's verdict and continued imprisonment. These include records suggesting the FBI coerced witnesses and excluded and falsified evidence.

Even the top prosecutor in the case, former US Attorney James Reynolds, has called for Peltier's release, as has retired FBI special agent Coleen Rowley, who last year described the bureau's continued opposition as having the appearance of an "emotion-driven 'FBI Family' vendetta."

President Joe Biden has the authority to grant clemency to Peltier without congressional approval, but has so far ignored overwhelming demands to do so.

Peltier is now 79 years old and in ailing health as his fight for freedom and justice continues.