OKLAHOMA CITY (AP) — The U.S. Supreme Court has rejected the final appeal of an Oklahoma death row inmate convicted in the 2004 death of a security guard.

The Oklahoman reports that justices declined to review Wade Lay's case on Monday. The 57-year-old was convicted of fatally shooting 36-year-old Air Force veteran Kenny Anderson.

Anderson was killed during an armed bank robbery in Tulsa involving Lay and his son, Christopher Lay. The son was sentenced to life in prison without parole.

Oklahoma hasn't carried out executions for more than three years after experiencing problems with lethal injection protocol.

Attorney General Mike Hunter and Corrections Department Director Joe Allbaugh say Oklahoma is developing a protocol to establish execution by inert gas. They say executions could resume by the end of this year at the earliest.