The last of three high-profile prisoners known as the Angola Three could walk free within days after a US federal judge ordered state officials to release him immediately.
Albert Woodfox, 68, has been in solitary confinement for 43 years but district judge James Brady ruled that he must now be set free.
Woodfox was a Black Panther activist seeking better conditions at the notorious Louisiana state prison called Angola in 1972 when a 23-year-old guard was stabbed to death.
He was one of three Black Panther leaders accused of the guard's death. He has been convicted twice at trial, but both convictions were overturned.
Human rights groups are closely monitoring the case.
Although the judge's ruling opens the way for Woodfox's freedom, Louisiana attorney general James "Buddy" Caldwell says he will appeal against it.