A man on Missouri's death row is on "an emotional roller coaster" amid efforts to spare his life with just one day before his scheduled execution, his attorney said.

Michael Tisius, 42, is set to die by injection Tuesday evening at the state prison in Bonne Terre for killing two jailers in 2000 in an ill-fated effort to free a man from a county jail. He would be the third Missouri man, and the 12th nationally, put to death in 2023.

A clemency request before Republican Gov. Mike Parson focuses on several issues. Among them: Tisius was just 19 at the time of the killings; he had been neglected as a child; and a juror at his 2010 resentencing may have been illiterate — in violation of Missouri law. It's unclear when Parson will announce his decision. A message left Monday with his spokeswoman wasn’t immediately returned.

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The U.S. Supreme Court on Monday declined to halt the execution based on Tisius' age when the crime occurred. A federal judge last week stayed the execution over the claim that a juror was illiterate, but an appeals panel reinstated it. The Supreme Court hasn't yet ruled on that issue.

Elizabeth Unger Carlyle, an attorney for Tisius, said the ups and downs of the appeals are taking a toll on him.

"I think he’s sort of, frankly, on an emotional roller coaster," Carlyle said. "He’s pretty anxious. He doesn’t want to die. I think he’s angry and frightened."

A 2005 Supreme Court ruling prohibits executions for those who were under 18 at the time of the crime. But Carlyle said "emerging science plus information about Mr. Tisius' own brain dictates that they should now change that rule to apply to Mr. Tisius."

A court filing from the Missouri attorney general's office noted that both the original trial jury and the jury at resentencing considered Tisius' age and mental health, "yet both juries still decided to impose the death penalty." The Supreme Court turned aside the appeal without comment.

Advocates for Tisius say he was largely neglected as a child and was homeless by his early teens. In 1999, as an 18-year-old, he was jailed on a misdemeanor charge for pawning a rented stereo system.

Michael Tisius

Michael Tisius, who in 2000 gunned down two prison guards during a failed jailbreak attempt, is scheduled to be executed Tuesday night. (AP Photo/Jeff Roberson, File)

In June 2000, Tisius was housed at the small Randolph County Jail in Huntsville with Roy Vance. Tisius was about to be released, and court records show the men discussed a plan in which Tisius would help Vance escape.

Just after midnight on June 22, Tisius went to the jail accompanied by Vance's girlfriend, Tracie Bulington. They told officers Leon Egley and Jason Acton that they were delivering cigarettes to Vance. The jailers didn't know that Tisius had a pistol.

At trial, Bulington testified that she looked up and saw Tisius with the gun drawn, then watched as he shot and killed Acton. When Egley approached, Tisius shot him, too. Both officers were unarmed.

Tisius found keys at the dispatch area and tried to open Vance's cell, but couldn't. When Egley grabbed Bulington's leg, Tisius shot him several more times.

Tisius and Bulington fled but their car broke down in Kansas. They were arrested in Wathena, Kansas, about 130 miles west of Huntsville. Tisius confessed to the crimes.

Bulington and Vance are serving life sentences.

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Defense attorneys have argued that the killings were not premeditated. Tisius, they said, intended to order the jailers into a holding cell and free Vance and other inmates. Tisius’ defense team issued a video last week in which Vance said he planned the escape attempt and manipulated Tisius into participating.

The people executed in Missouri this year included Amber McLaughlin, who killed a woman and dumped the body near the Mississippi River in St. Louis. The execution was believed to be the first of a transgender woman in the U.S.

Raheem Taylor, 58, was put to death in February for killing his live-in girlfriend and her three children in 2004 in St. Louis County.

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Four of the U.S. executions this year have been in Texas, and three in Florida.