A federal court Friday blocked the release of Albert Woodfox, the last incarcerated member of a group of Louisiana prisoners known as the Angola Three, and raised doubts as to whether Woodfox will avoid a third trial in the 1972 slaying of a prison guard.

Woodfox must now remain behind bars until at least late August, when the 5th U.S. Circuit Court of Appeals hears arguments on the state's appeal of an judge's order denying a third effort to make a conviction stand in the stabbing death of Brent Miller.

In a blow to his defense, the appellate panel's unanimous ruling suggested that Louisiana might be able to try Woodfox again after all, despite the earlier judge's conclusion that it would be impossible to guarantee a fair trial, given that 43 years have passed, key witnesses have died and there is no physical evidence linking Woodfox to the crime.

Louisiana "has made a strong showing that it is likely to succeed on the merits of the appeal," the panel wrote. "No showing has been made that any state retrial (or any appeal) will be improperly handled."

The decision caps a tumultuous week in a decades-long case that has focused international attention on the use of solitary confinement in American prisons.

Judge James Brady took the extraordinary step of barring a third trial for Woodfox, whose previous convictions were overturned for reasons including racial bias in selecting a grand jury foreman and juror misconduct. "There is no valid conviction holding him in prison, let alone solitary confinement," Brady ruled on Monday.

But by Friday, the appellate judges ordered Woodfox held until at least the last week of August, when a hearing was scheduled on the state's appeal.

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. Prison authorities said their Black Panther Party activism would otherwise rile up other inmates at the maximum-security prison farm in Angola.

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

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

But 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, and couldn't be cross-examined about whether his testimony was induced by favorable treatment.

The defense also argued this week that Woodfox, now 68 and in ill health, poses no danger to the community if freed during the appeal.

But the appellate panel, in a 10-page order, ruled otherwise.

"There is a substantial interest in staying the release of a person, twice convicted of murder, from being released from a life sentence without the possibility of parole," Judge Jerry E. Smith wrote.

The Louisiana attorney general's office maintains that Woodfox is too dangerous to set free.

"It has always been the State's priority to ensure justice for the brutal slaying of Brent Miller and to hold accountable this murderer who has an extensive history of violent crimes," said Aaron Sadler, a spokesman for the attorney general.

Outside the jail where Woodfox was held Friday, about a dozen members of the guard's family had been waiting for a ruling, along with dozens of reporters and at least eight television crews. "Praise God," some of the guard's family members said when the word came down.

"Whenever you murder somebody or do something wrong, you pay the price," said one of Miller's sisters, Wanda Callender. "Two juries said he was guilty... I would love for him to stay in prison the rest of his life."

Miller's widow, Teenie Rogers, was not at the jail Friday, but she has pressed for Woodfox's release, saying she no longer believes he was responsible.

"We are deeply disappointed that after 40 years of incarceration under the harshest conditions possible, Mr. Woodfox will not be released today," said Carine Williams, a Woodfox lawyer.

She declined to describe how Woodfox took it when she broke the news to him.

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Associated Press reporters Kevin McGill and Cain Burdeau in New Orleans contributed to this story.