Washington (CNN) -- The execution of a Utah death-row inmate by firing squad moved a step closer Tuesday after the state's high court denied his latest appeal.
Unless the U.S. Supreme Court intervenes, Ronnie Lee Gardner is set to die early Friday for the 1985 slaying of attorney Michael Burdell. The killing took place during Gardner's attempt to escape from custody at a Salt Lake City courthouse.
The state justices unanimously declined Gardner's request for a new sentencing hearing, saying he waited too late to make his latest claims for relief.
"All of the claims Mr. Gardner raises in his most recent petition for post-conviction relief are claims that he could have raised more than a decade ago," said Associate Chief Justice Matthew Durrant.
The court also noted various state courts have "endeavored scrupulously" to protect the 49-year-old prisoner's rights.
Gardner would become only the third person to die by rifle fire, all in Utah, since the Supreme Court restored the death penalty in 1976. Tuesday, his lawyers filed the first of what could be several federal appeals in coming days.
On Monday, Utah's Board of Pardons and Parole refused to commute the twice-convicted killer's death sentence. The commission members were not persuaded by Gardner's pleas during a two-day hearing last week that he is a changed man.
Gardner had a long history of escapes and was slipped a gun before he fatally shot Burdell on April 2, 1985. He was there for a pretrial 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.
A last-minute appeal for stay of execution is expected to be filed with the U.S. Supreme Court. The execution is set for midnight (2 a.m. ET) at the Utah State Prison in Draper, about 20 miles south of Salt Lake City.
During the commutation hearing, parole board members heard testimony regarding Gardner's childhood, which was punctuated by poverty, abuse and neglect. His attorney, Andrew 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 has firing squad as a current execution method. Oklahoma allows it only if lethal injection and electrocution are ruled unconstitutional. Forty of the 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 each use matching .30-caliber rifles, standing behind a wall cut with five gunports. One of the rifles will hold an "ineffective" round, similar to a blank, which delivers the same recoil as a live round.
The marksmen fire from a distance of 25 feet. The inmate is blindfolded and strapped to a chair with a target pinned to his chest.
The state court case is Utah v. Gardner (No. 20100345).
CNN's Ashley Hayes contributed to this report.