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, raising a clenched fist as he walked free.

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 30 miles north of Baton Rouge.