A Missouri man who shot and killed two jailers nearly 23 years ago during a failed bid to help an acquaintance escape from a rural jail was executed Tuesday evening.

Michael Tisius, 42, received a lethal injection of pentobarbital at the state prison in Bonne Terre and was pronounced dead at 6.10pm, authorities said. He was convicted of the June 22, 2000, killings of Leon Egley and Jason Acton at the small Randolph County Jail.

In a final written statement, Tisius said he tried hard “to become a better man” and he expressed remorse for his crimes.

“I am sorry,” he wrote. “And not because I am at the end. But because I truly am sorry.”

Tisius’ lawyers had urged the US Supreme Court to block the execution, alleging in appeals that a juror at a sentencing hearing was illiterate, in violation of Missouri law. The court rejected that motion Tuesday afternoon.

The Supreme Court previously turned aside another argument — that Tisius should be spared because he was just 19 at the time of the killings. A 2005 Supreme Court ruling bars executions of those under 18 when their crime occurred, but attorneys for Tisius had argued that even at 19, when the killings occurred, Tisius should have had his sentence commuted to life in prison without parole.

Advocates for Tisius had said he was largely neglected as a child and was homeless by his early teens. His path to the death chamber began in 1999 when, as an 18-year-old, he was jailed on a misdemeanour charge of pawning a rented stereo system.

This booking photo provided by the Missouri Department of Corrections shows Michael Tisius. Photo / AP

In June 2000, Tisius was housed on the misdemeanour charge at the same county jail in Huntsville with inmate Roy Vance. Tisius was about to be released, and court records show the men discussed a plan in which Tisius, once he was out, would help Vance escape.

Just after midnight on June 22, 2000, Tisius went to the jail accompanied by Vance’s girlfriend, Tracie Bulington. They told Egley and Acton that they were there to deliver 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 later that day in Kansas. They were arrested in Wathena, Kansas, about 210km west of Huntsville. Tisius confessed to the crimes.

Bulington and Vance are serving life sentences on murder convictions.

Defence 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’ defence team issued a video last week in which Vance said he planned the escape attempt and manipulated Tisius into participating.

Republican Gov. Mike Parson on Monday rejected a petition for clemency, saying in a statement: “It’s despicable that two dedicated public servants were murdered in a failed attempt to help another criminal evade the law.”

The execution was the 12th in the US this year, and the third in Missouri. Only Texas, with four, has executed more people than Missouri this year.

Amber McLaughlin, 49, who killed a woman and dumped the body near the Mississippi River in St. Louis, was put to death in January. The execution was believed to be the first of a transgender woman in the US. Raheem Taylor, 58, was executed in February for killing his live-in girlfriend and her three children in 2004 in St Louis County.

Another Missouri execution is scheduled for August 1. Johnny Johnson was convicted of sexually assaulting and killing a 6-year-old girl in St Louis County in 2002.

- Jim Salter, AP