OKLAHOMA CITY (AP) - Oklahoma is set to execute the fourth death-row inmate of 2012, a 39-year-old man who was convicted of three counts of first-degree murder in the December 1993 shooting deaths of his former girlfriend and her two young children.

Michael Hooper is scheduled to receive a lethal dose of drugs Tuesday evening at the Oklahoma State Penitentiary in McAlester for the killings of 23-year-old Cynthia Lynn Jarman, and her two children, 5-year-old Tonya and 3-year-old Timmy.

Hooper has challenged the state's execution protocol in a lawsuit that seeks a stay of his execution. A final appeal is pending in the U.S. Supreme Court.

Three other death row inmates - Gary Roland Welch, Timothy Stemple and Michael Selsor - have been executed so far this year.