A member of the Black Panthers who has spent 43 years in solitary confinement – the longest period in American history – has been released at the age of 69. Albert Woodfox, one of a group known as the Angola Three, had been kept apart from other prisoners in a Louisiana jail since April 1972, until he was released last night.

Woodfox and two others, Herman Wallace and Robert King, were linked to the death of prison guard Brent Miller at the Louisiana State Penitentiary at Angola. All three maintained their innocence, but Woodfox and Wallace were convicted of Miller's murder.

Woodfox had his conviction overturned twice during decades of legal battles, but remained behind bars as the state looked to try him a third time. At the age of 69, however, he agreed to accept lesser charges of manslaughter, which lead to his release.

"Although I was looking forward to proving my innocence at a new trial, concerns about my health and my age have caused me to resolve this case," he told the media.

#FreeAlbertWoodfox New photo of Albert Woodfox following his release, at Democracy Now! https://t.co/qMQkwxprWq pic.twitter.com/EYJJOOaDro — Angola 3 News (@angola3news) February 19, 2016

Activists have long argued that there is no evidence to tie the three men to the crime, and criticised the length of the solitary confinement, which was renewed every 90 days for Woodfox's 43 years of incarceration. Wallace was released in 2013, days before dying of terminal liver cancer, while King was freed in 2001 after a conviction for killing another inmate was overturned.

A member of the black rights group the Black Panthers, Woodfox was in prison for armed robbery and assault when Miller was killed. Woodfox and his supporters believe the Angola Three were framed for the murder in order to prevent them from organising a Blank Panther Party chapter inside the prison in the early 1970s.

"I want to thank my brother Michel for sticking with me all these years, and Robert King, who wrongly spent nearly 30 years in solitary. I could not have survived without their courageous support, along with the support of my dear friend Herman Wallace, who passed away in 2013," Woodfox said upon his release. "I also wish to thank the many members of the International Coalition to Free the Angola 3, Amnesty International, and the Roddick Foundation, all of whom supported me through this long struggle."

Jasmine Heiss, a spokesperson for Amnesty USA, said: "After four decades of isolation, Albert Woodfox's release is long overdue and undeniably just.

"Nothing will truly repair the cruel, inhuman and degrading solitary confinement that the state of Louisiana inflicted upon him. But this belated measure of justice, on Woodfox's 69th birthday, is something he has been seeking for more than half his life."