An Oklahoma death row inmate's execution was stayed for the third time in less than three months Tuesday, after his defense attorneys objected to plans to substitute a drug in his lethal injection.

U.S. District Judge Stephen Friot issued the stay just hours before Jeffrey David Matthews, 38, was set to be executed for the murder of his 77-year-old great-uncle, Otis Earl Short, during a 1994 robbery of Short's home.

Friot stayed the execution after defense attorneys protested Department of Corrections plans to substitute the first of three drugs used in the lethal injection process, said Charlie Price, spokesman for Attorney General Drew Edmondson's office.

Rather than using sodium thiopental, an anesthetic capable of rendering a prisoner unconscious in a matter of seconds, prison officials wanted instead to use a drug called Brevital, a form of methohexital sodium, Price said. Corrections officials were concerned the original drug on hand may have been expired.

"Corrections had found a substitute drug the state believed complied with protocol," Price said.

The state's attorneys agreed substituting the drugs complied with Oklahoma's statute for lethal injection protocol but defense attorneys objected to the switch, Price said.

Defense attorney Robert Jackson of the Federal Public Defender's Office did not immediately return a telephone call seeking comment.

Gov. Brad Henry already had granted two stays since the Oklahoma Pardon and Parole Board denied Matthews clemency May 26, giving defense attorneys time to investigate their client's claims of innocence and review fingerprint evidence.

Friot's order Tuesday stayed Matthews' execution another 60 days and scheduled an Oct. 15 hearing on a preliminary injunction sought by defense attorneys that could further extend the stay, Price said.

Prison officials believe the drugs they intended to substitute are essentially the same and would have the same effect in rendering a prisoner unconscious before other lethal drugs are administered, said Department of Corrections spokesman Jerry Massie. The other two drugs in Oklahoma's lethal injection protocol are vecuronium bromide, which stops respiration, and potassium chloride, which stops heart activity.

Massie said prison officials had an ample supply of sodium thiopental on hand, but it was possible the chemical was out of date.

"We had had it for a while," Massie said. "When we had it tested it lacked the purity that we wanted. It's not something you want to take any chance on."

Matthews was convicted of being one of two men who stormed into Short's McClain County home east of Rosedale on Jan. 27, 1994. Trial testimony indicated Matthews shot Short once in the head at close range with a .45-caliber pistol and that the second man, Tracy Dyer, cut the throat of his wife, Minnie Short. Minnie Short survived the attack.

Dyer, 36, was convicted of first-degree murder and other charges and was sentenced to life in prison without the possibility of parole.