Mr. Woodfox, who was serving a sentence for armed robbery at the time of the murder, would most likely have been released from solitary many years ago if he had pleaded guilty to the murder. But he has consistently denied any involvement, believing that he was targeted because of his political activism as a member of the Black Panther Party.

The facts of the case were on his side: There was no physical evidence linking him or his co-defendant, Herman Wallace, to the murder, and prosecutors did not reveal that their main witness had been bribed to testify against the men. Mr. Woodfox, by all accounts, has been a model prisoner, and under Louisiana prison policy this should have earned him his exit from solitary confinement years ago.

Finally, on Thursday, a federal appeals court panel in New Orleans unanimously voted to overturn Mr. Woodfox’s murder conviction because his 1998 retrial was tainted by racial bias in the grand jury selection process. (His original, 1973 conviction was overturned because of his lawyer’s ineffectiveness. In 2008, a federal judge overturned his second conviction on grounds of bad lawyering, but that ruling was reversed because of a federal law that dramatically restricts review of state court decisions.)

Mr. Woodfox is the last incarcerated member of what became known as the Angola 3 — three prisoners who each spent decades in solitary, mostly at Louisiana’s notorious Angola prison. Robert King, who was put into isolation also in 1972, was released in 2001. Mr. Wallace was released last year, and he died of liver cancer days later.

In 2005, a federal magistrate judge wrote in a report that the amount of time the men had spent in solitary was “so far beyond the pale” that she could not find “anything even remotely comparable in the annals of American jurisprudence.” Yet in 2008, 36 years after the guard’s killing, the Louisiana attorney general was still calling Mr. Woodfox “the most dangerous person on the planet.”