ATLANTA - A Georgia death-row inmate convicted of killing a fellow prisoner is set to be executed later this month, the state attorney general's office said Wednesday.

The commissioner of the Department of Corrections on Wednesday set the execution of Warren Lee Hill for July 15 at 7 p.m., the office announced in a news release. Earlier Wednesday, a judge had signed a warrant setting a window for the execution between noon on July 13 and noon on July 20.

Hill's lawyers in May asked the U.S. Supreme Court to review his case in light of new evidence they have submitted. The court has not yet responded.

The attorneys have long argued that Hill is mentally disabled and therefore should not be put to death because the execution of mentally disabled offenders is prohibited by state law and a 2002 U.S. Supreme Court decision. But the state has consistently argued that Hill's lawyers have failed to prove beyond a reasonable doubt that he is mentally disabled.

Hill was sentenced to die for the 1990 beating death of fellow inmate Joseph Handspike. Hill bludgeoned Handspike with a nail-studded board while his victim slept, authorities said. At the time, Hill was already serving a life sentence for the 1986 slaying of his girlfriend, Myra Wright, who was shot 11 times.

Hill has come within hours of death on two previous dates - in July 2012 and in February of this year - before the scheduled executions were halted by last-minute court orders.

A three-judge panel of the 11th U.S. Circuit Court of Appeals in February temporarily stayed Hill's execution to give him a chance to argue that a federal court should reconsider his case based on new statements by mental health experts. But the panel later ruled Hill was procedurally barred from submitting his case to a federal court for reconsideration.

Three state experts who testified in 2000 that Hill was not mentally disabled wrote sworn statements in February saying they were rushed in their evaluation at the time, that they now have more experience and that there have been scientific developments since then. All three reviewed facts and documents in the case and write that they now believe Hill is mentally disabled.

"All experts who have evaluated Warren Hill agree: he is mentally retarded," Hill's lawyer Brian Kammer said in an email. "This case presents the extraordinary circumstance where an individual who is ineligible for a capital sentence is about to be executed. Mr. Hill has no recourse left but to beg the nation's highest court to intervene, and we trust and hope that the U.S. Supreme Court will hear his plea."

The state's supply of the execution drug pentobarbital expired in March, and the drug has become increasingly difficult for states to get since the manufacturer has said it doesn't want the drug used in executions. A Department of Corrections spokesman did not immediately respond to an email and phone messages seeking comment about the state's execution drug supply.