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[Updated at 9:32 p.m. ET] The U.S. Supreme Court has denied a stay of execution for Utah inmate Ronnie Lee Gardner, who is scheduled to be executed by firing squad at 2 a.m. ET.

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[Updated at 5:56 p.m. ET] Utah Gov. Gary R. Herbert has declined inmate Ronnie Lee Gardner's request to temporarily stay Gardner's anticipated execution by firing squad. The execution is scheduled for 2 a.m. ET.

[Updated at 4:01 p.m. ET] The 10th Circuit Court of Appeals in Denver, Colorado, has denied the motion to delay the firing squad execution of convicted killer Ronnie Lee Gardner, which is scheduled for 2 a.m. Friday in Utah.

[Posted at 12:46 p.m. ET] Hours away from his scheduled execution by firing squad, Utah death-row inmate Ronnie Lee Gardner has asked Gov. Gary Herbert for a temporary stay, according to the state Department of Corrections.

The three-page letter was delivered to Herbert shortly before 10 a.m., the department said in a statement on its website. It was signed by Gardner's attorneys and asks the governor to "issue a respite or reprieve pursuant to your executive power under the Utah Constitution."

The request is currently under review, according to the statement.

In addition, the U.S. Supreme Court is expected to rule Thursday on Gardner's appeal for a last-minute stay of execution.

Gardner, 49, faces execution by firing squad shortly after midnight Friday (2 a.m. ET) for the shooting death of attorney Michael Burdell during a botched scape attempt from custody in 1985 at a Salt Lake City, Utah, courthouse. He would be only the third person to die by firing squad in the United States in 33 years.

Among the claims Gardner raises in his appeals is that he has been a death row inmate too long. "He asserts that executing him now, after nearly 25 years on death row in Utah, so lacks retributive or deterrent value that it violates the Eighth Amendment," Andrew Parnes, Gardner's lawyer, told the high court. He did not return phone calls from CNN seeking comment.

A federal judge late Tuesday refused to block the execution, after Gardner claimed the procedures related to a two-day commutation hearing held by the state Board of Pardons and Parole last week violated his civil rights. The parole board Monday refused to commute Gardner's sentence to life in prison, and the Utah Supreme Court on Tuesday also denied his request for a stay.

Gardner testified before the board he is a changed man, and his sentence should be commuted.

He is set to become the third person to die by rifle fire, all in Utah, since the Supreme Court restored the death penalty in 1976. He had a long history of escapes and was slipped a gun before he fatally shot Burdell on April 2, 1985. He was at the courthouse for a pre-trial hearing in the 1984 slaying of Melvyn Otterstrom, who was killed at the Salt Lake City bar where he was working to earn extra money.

The execution will be held at the Utah State Prison in Draper, about 20 miles south of Salt Lake City.

Corrections officials announced Gardner had consumed his last meal Tuesday night at 6:30 p.m., and has elected to fast prior to the anticipated execution. His dinner included steak, lobster, 7-Up, apple pie and vanilla ice cream.

During the commutation hearing, parole board members heard testimony regarding Gardner's childhood, which was punctuated by poverty, abuse and neglect. Parnes maintained that jurors in the Burdell trial never heard this evidence - and presented affidavits from jurors who said it might have persuaded them to decide against the death penalty.

Life in prison without the possibility of parole was not an option for jurors at the time, and Parnes said it was suggested to the jury that Gardner might be released from prison at some point if he were given a life sentence.

Gardner pleaded guilty to second-degree murder in Otterstrom's death, and jurors were not told of a judge's recommendation in that case that he not be released from prison, Parnes said.

Utah is the only state that uses the firing squad as a current execution method. Oklahoma allows it only if lethal injection and electrocution are ruled unconstitutional. Forty of Utah's 49 executions in the last 160 years or so have been done this way, according to the Death Penalty Information Center.

State lawmakers made lethal injection the default capital punishment in 2004, but at least three inmates who already had chosen the firing squad were grandfathered in under the new law.

Five anonymous marksmen will use matching .30-caliber rifles, standing behind a wall cut with five gunports. One of the rifles will be an "ineffective" round, similar to a blank, which delivers the same recoil as a live round. That ensures none of the riflemen will know who delivered the fatal shot.

The marksmen fire from a distance of 25 feet. The inmate is blindfolded, strapped to a chair and a target pinned to his chest.

The Supreme Court case is Gardner v. Utah (09-11378).

- CNN's Ashley Hayes and Bill Mears contributed to this report.