Leonard Peltier has been in prison for longer than he was a free man. He was convicted in 1977 of murdering two Federal Bureau of Investigation (FBI) agents in June 1975, despite serious concerns about the fairness of his trial.

Saturday marks his 40th year in US custody, the American Indian Movement (AIM) leader writes in his open letter published by CounterPunch.

The 71-year-old, who admits he’s “not sure I have much time left”, is still being held in a maximum security penitentiary.

"My 40 Years in Prison" by Leonard Peltier, political prisoner since February 6, 1976 - https://t.co/ORXhfzWp4Bpic.twitter.com/GyvTJZMVkw — Scott Campbell (@incandesceinto) February 6, 2016

“I have been classified as a medium security prisoner now for at least 15 years, and BOP (Bureau of Prisons) regulations say elders shall be kept in a less dangerous facility/environment. But NOT if you’re Leonard Peltier, I guess,” he laments.

Today marks 40 years of illegal/wrongful imprisonment for Leonard Peltier. We recorded this… https://t.co/oXZgNbAFR4 — Last Internationale (@TLImusic) February 6, 2016

25 years ago, the judge who presided over his case wrote a letter to the chair of the Senate Select Committee on Indian Affairs with concerns that should “merit consideration in any petition for leniency filed”, according to the Indian Country Today Media Network.

Despite this, Peltier writes this week that, “In all of the applications for clemency, the FBI has interfered with an executive order.”

Peltier reflects on his previous bids for clemency including a long-forgotten story about Republican president Ronald Reagan’s broken promise.

AIM and Peltier’s case gained international attention, particularly in the Soviet Union during the late 1980s, when Russia used Peltier’s case as an example of human rights abuses in the US.

Some Truth to It? How Soviet Media Used Leonard Peltier as Anti-U.S. Propaganda [Video] http://t.co/v774kzCF20 — Indian Country Today (@indiancountry) March 20, 2015

Russell Red Bear, who was present at Reagan’s Moscow meeting with his Russian counterpart, brought Peltier a copy of "Perestroika" personally signed by Mikhail Gorbachev.

Peltier writes in his new letter that the US president promised Gorbachev “he would release him if the Soviet Union released a prisoner, but Reagan reneged”.

His letter then takes a sobering turn when he reveals a serious medical condition.

“Today, I’m facing another dilemma — an abdominal aortic aneurysm (AAA),” Peltier writes. “The doctor told me if it bursts, I can bleed to death. It’s also close to my spine and I could end up paralyzed. The good news is that it’s treatable and the operation has a 96-98 percent success rate. BUT I’m in a max security prison. We don’t get sent for treatment until it is terminal.”

He calls on President Obama to “have the courage and conviction to commute my sentence and send me home to my family.”

Before his death in 2013, South African president Nelson Mandela added himself to the considerable list of celebrities who have called for Peltier’s executive clemency.

LEONARD PELTIER - Today marks the 40th year of Leonard's incarceration, that's 13 years longer than Nelson Mandela https://t.co/ySOlz0v6HJ — ballymurphy11 (@ballymurphy11) February 7, 2016

Six years ago, folk singer Pete Seeger, actor and musician Harry Belafonte, hip-hop artist Common, and filmmaker Michael Moore came together for a benefit concert in New York City, calling on Obama to release Peltier.

Seeger described his involvement with the movement to free Peltier on the radio program “Smiley & West”.

Peltier concluded his letter with a thank you to his supporters around the world: “You all worked hard — are still working hard — to spread the word about what is now being called the most outrageous conviction in U.S. history. If I should be called home, please don’t give up on our struggle. In the Spirit of Crazy Horse…”

Leonard Peltier, 40 years in jail #FreeLeonardPeltier@Ireland4Peltier about time he was given his freedom pic.twitter.com/sAxUuTCcW2 — Kevin McKeogh (@KevinFXL) February 6, 2016