A man who abducted a 6-year-old Missouri girl and beat her to death at an abandoned factory two decades ago was put to death on Tuesday evening, shortly after the US Supreme Court rejected a request to block the execution over arguments he was mentally incompetent.

Johnny Johnson, 45, received a lethal injection dose of pentobarbital at a state prison in Bonne Terre and was pronounced dead at 6.33pm CDT, authorities said.

He was convicted of the July 2002 killing of Casey Williamson in the St Louis area suburb of Valley Park.

Johnson, who had schizophrenia, expressed remorse in a brief handwritten statement released by the Department of Corrections hours before being executed. “God Bless. Sorry to the people and family I hurt,” Johnson’s statement said.

As he lay on his back with a sheet up to his neck, Johnson turned his head to the left, appearing to listen to his spiritual adviser shortly before the injection began. He then faced forward with his eyes closed, with no further physical reaction.

Among those witnessing Johnson’s execution were several members of the girl’s family and the former prosecutor and police investigator who handled his case.

The US Supreme Court, with Justice Sonia Sotomayor and two other justices dissenting, rejected a late request to stay the execution.

In recent appeals, Johnson’s attorneys have said the inmate has had delusions about the devil using his death to bring about the end of the world.

Casey Williamson was slain by Johnny Johnson in Valley Park, Missouri, in 2002. Photo / Supplied

“The Court today paves the way to execute a man with documented mental illness before any court meaningfully investigates his competency to be executed,” Sotomayor and the other dissenting justices wrote in a statement when the stay was rejected.

“There is no moral victory in executing someone who believes Satan is killing him to bring about the end of the world.”

Former St Louis County Prosecutor Bob McCulloch called the delusions “nonsense” and said Johnson inflicted “unspeakable horrors” upon Casey.

”He’s got some issues - significant issues,” McCulloch said moments before witnessing the execution. But “he knew exactly what he was doing”.

The girl’s disappearance from her hometown of Valley Park on July 26, 2002, had set off a frantic search before her body was found.

Casey’s mother had been best friends in childhood with Johnson’s older sister and even helped babysit him.

Johnson, 45, was convicted of first-degree murder in in the 2002 death of Casey Williamson in suburban St Louis. Photo / AP

After Johnson attended a barbecue the night before the killing, Casey’s family let him sleep on a couch in the home where they also were sleeping. In the morning, Johnson lured the girl — still in her nightgown — to the abandoned glass factory, even carrying her on his shoulders on the walk to the dilapidated site, according to court documents.

When he tried to sexually assault her, Casey screamed and tried to break free. He killed her with a brick and a large rock, then washed off in the nearby Meramec River.

Johnson confessed that same day to the crimes, according to authorities.

“It was more violent and brutal than any case I’ve ever seen,” said former St Louis County homicide investigator Paul Neske, who questioned Johnson at length the day of Casey’s murder and witnessed his execution.

After a search by first responders and volunteers, Casey’s body was found in a pit, buried under rocks and debris, less than kilometre from her home.

At Johnson’s trial, defence lawyers presented testimony showing their client - an ex-convict who had been released from a state psychiatric facility six months before the crime - had stopped taking his schizophrenia medication and was acting strangely in the days before the slaying.

In June, the Missouri Supreme Court denied an appeal seeking to block the execution on arguments that Johnson’s schizophrenia prevented him from understanding the link between his crime and the punishment. A three-judge federal appeals court panel last week temporarily halted execution plans, but the full 8th US Circuit Court of Appeals reinstated it.

Johnson’s attorneys then filed appeals to the US Supreme Court centered around his competency to be executed.

Governor Mike Parson on Monday denied a request to reduce Johnson’s sentence to life in prison.

The clemency petition by Johnson’s attorneys said Casey’s father, Ernie Williamson, opposed the death penalty. But Casey’s great aunt, Della Steele, wrote an emotional plea to the governor urging the execution to be carried out to “send the message that it is not okay to terrorise and murder a child”.

Steele said grief from Casey’s death led to destructive effects among other family members.

“He did something horrible. He took a life away from a completely innocent child, and there have to be consequences for that,” Steele said recently, speaking with The Associated Press.

The family has organised community safety fairs in Casey’s memory, including a July 22 event that drew a couple of hundred people.

The family gave away dozens of child identification kits along with safety tips involving fire, water and bicycles, among other items.

The execution was the 16th in the US this year, including three previously in Missouri, five in Texas, four in Florida, two in Oklahoma and one in Alabama.

”It’s been a difficult day, and a difficult 21 years,” Steele said in a statement after witnessing the execution. “We will continue to honor our sweet Casey’s memory by doing our best to make a difference in the lives of other children.”