WASHINGTON (AP) - The Supreme Court has refused to halt the execution of an Oklahoma inmate who claims he is mentally incompetent and ineligible for the death penalty.

The justices said in an order Tuesday that they will not grant a stay of execution to 56-year-old Garry Thomas Allen, who is set to die by lethal injection Tuesday evening at the Oklahoma State Penitentiary in McAlester.

Allen was convicted in the shooting death of his fiance outside a children's day care in Oklahoma City in 1986.

Allen's attorneys had argued that he was incompetent when he entered a guilty plea in the case and that his mental condition has continued to deteriorate. Appellate courts have rejected those claims.