The 9th U.S. Circuit Court of Appeals ordered that Steven Craig James be resentenced because he received inadequate legal representation during his 1981 sentencing in Maricopa County Superior Court.
The appellate court had issued a similar ruling in 2012, but it considered James' case again at the order of the U.S. Supreme Court.
Under the 9th Circuit's ruling Friday, James either must be resentenced or his death sentence automatically becomes a life prison term.
James was convicted in the 1981 killing of Juan Maya. The victim was attacked by James and two other men who shot and struck Maya with rocks before throwing his body down an abandoned mine shaft.