ST. FRANCISVILLE, La. -- The last inmate of a group known as the "Angola Three" pleaded no contest Friday to manslaughter in the 1972 death of a prison guard and was released after more than four decades in prison.

Albert Woodfox and two other men became known as the "Angola Three" for their decades-long stays in isolation at the Louisiana Penitentiary at Angola and other prisons. Their cases drew condemnation from human rights groups and focused attention on the use of solitary confinement in American prisons.

Officials said they were kept in solitary because their Black Panther Party activism would otherwise rile up inmates at the maximum-security prison farm in Angola.

Woodfox consistently maintained his innocence in the killing of guard Brent Miller. He was being held at the West Feliciana Parish Detention Center in St. Francisville, about 50 kilometres north of Baton Rouge. He was awaiting a third trial in Miller's death after earlier convictions were thrown out by federal courts for reasons including racial bias in selecting a grand jury foreman.

Woodfox, who turned 69 on the same day he was released from custody, spoke to reporters and supporters briefly outside the jail before driving off with his brother. Speaking of his future plans, he said he wanted to visit his mother's gravesite. She died while he was in prison, and Woodfox said he was not allowed to go to the funeral.

As to whether he would have done anything differently back in 1972, Woodfox responded: "When forces are beyond your control, there's not a lot you can do. Angola was a very horrible place at the time and everybody was just fighting to survive from day to day."

In a press release earlier Friday, Woodfox thanked his brother and other supporters who have lobbied over the years for 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 now and obtain my release with this no contest plea to lesser charges. I hope the events of today will bring closure to many," he said.

He had been twice convicted of murder and pleaded guilty Friday to manslaughter and aggravated burglary.

At the time of Miller's killing, Woodfox was serving time for armed robbery and assault. Inmates identified him as the one who grabbed the guard from behind while others stabbed Miller with a lawnmower blade and a hand-sharpened prison knife.

The star witness, a serial rapist who left death row and was pardoned by the Louisiana governor after his testimony, died before the second trial.

Woodfox was placed in solitary immediately after Miller's body was found in an empty prison dormitory, and then was ordered kept on "extended lockdown" every 90 days for decades.

A "no contest" plea is not an admission of guilt, and Woodfox has long maintained his innocence. But it does stand as a conviction, and Woodfox was freed after being given credit for time served.

Louisiana Attorney General Jeff Landry said the plea deal was "in the best interest of justice," and was reached with the co-operation of Miller's family.

Woodfox made his first public appearance at an event in New Orleans with Black Panther Party members and supporters Friday night. Dressed all in black, with grey hair, he looked both overwhelmed and physically tentative with a quiet, stunned joy in his eyes. The event turned into a celebration party with cheers, happy birthday wishes, hugs and emotion. He embraced supporters, friends and the other Angola Three member still alive, Robert King.

But he kept his remarks short and declined an interview request by The Associated Press.

"I hope you understand that I've been through a terrible ordeal and I need a little time to get my footing so I don't make a fool of myself," he said.

In addition to visiting his mother's grave, Woodfox's future plans include getting a full medical checkup, said one of his lawyers, George Kendall. Woodfox has had numerous ailments, including a form of hepatitis, and has lacked quality medical care during his imprisonment.

Woodfox's brother, Michael Mable, said his brother was doing well, and taking life one day at a time: "He'll think about tomorrow, tomorrow. It's a whole new process."

The other Angola Three inmates was Herman Wallace, who died a free man in October 2013, just days after a judge granted him a new trial in Miller's death. King was released in 2001 after his conviction in the death of a fellow inmate was overturned.