HARRISBURG, Pa. (Reuters) - A Philadelphia judge has allowed convicted police-killer Mumia Abu-Jamal to launch a new appeal, ruling that a former chief justice of the Pennsylvania Supreme Court should have recused himself from the inmate’s previous appeals.

Abu-Jamal, 64, a former black nationalist and public radio reporter, was sentenced to death for the 1981 killing of Philadelphia police officer Daniel Faulkner. His sentence was commuted to life without parole in 2011.

Judge Leon W. Tucker said former Chief Justice Ronald Castille should have stepped aside because his past service as Philadelphia District Attorney — the office which prosecuted Abu-Jamal — created an appearance of bias.

“True justice must be completely just without even a hint of partiality, lack of integrity, or impropriety,” Tucker wrote in his opinion issued late on Thursday. “Petitioner is entitled to an unbiased tribunal, without even the appearance of impropriety.”

Lawyers for Abu-Jamal now have 30 days to begin a new appeal, which would first be heard by the Pennsylvania Superior Court and then, if necessary, by the Supreme Court. Castille, who could not be reached for comment on Friday, retired in 2014. Four new justices have joined the seven-member high court since Abu-Jamal’s last appeal was denied in 2012.

Abu-Jamal, a past president of the Philadelphia Association of Black Journalists, sought permission for a new appeal after the U.S. Supreme Court ruled in 2016 that Castille’s refusal to recuse himself from an appeal of a different death penalty case created an impermissible risk of bias.

Tucker said Abu-Jamal was unable to prove Castille’s direct involvement in his prosecution and death sentence — he was then an assistant district attorney — but noted Castille campaigned for the Supreme Court in 1993 touting the number of defendants he sent to death row.

“This was a straightforward application of federal and Pennsylvania law requiring cases to be decided by judges whose impartiality cannot reasonably be questioned,” said Judith Ritter, one of Abu-Jamal’s attorneys.

Dustin Slaughter, a spokesman for the Philadelphia District Attorney’s office, said they are examining the ruling and have not yet decided whether to appeal.

Abu-Jamal was once described by the New York Times as the world’s best-known death row inmate. Tucker called his case “one of the most polarizing criminal cases in Philadelphia history, the nation, and perhaps worldwide.”

Maureen Faulkner, the officer’s widow, was removed from Tucker’s courtroom in October after an outburst during the hearings that preceded his ruling.