William Barnes shot and paralyzed police Officer Walter Barclay in 1966 but was charged with murder and jailed after Barclay died in August 2007.
A jury exonerated Barnes in May 2010 but the case’s lead prosecutor vowed to fight his release. His requests for parole have been repeatedly denied.
His attorney on Monday argued in federal court that Barnes is in effect serving time for a crime of which he was acquitted.
The state attorney general’s office said the parole board has followed proper procedures in making its decisions.