A Missouri man who abducted a 6-year-old girl and beat her to death after luring her to an abandoned factory more than 20 years ago was executed on Tuesday evening. 

Johnny Johnson, 45, was put to death by lethal injection of pentobarbital at a state prison in Bonne Terre shortly after the Supreme Court rejected a request to block his execution over claims he was mentally incompetent.

Johnson was convicted of the July 2002 killing of Casey Williamson in Valley Park, a suburb of St. Louis. 

The Department of Corrections released a handwritten statement from Johnson hours before his execution in which he expressed remorse.

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"God Bless. Sorry to the people and family I hurt," Johnson said.

Shortly before the injection began, Johnson was lying on his back and turned his head to the left, appearing to listen to his spiritual adviser. He then faced forward with his eyes closed and showed no further physical reaction.

Several members of the girl’s family and the former prosecutor and police investigator who handled his case were present for the execution.

The Supreme Court, in a six to three decision, rejected a late request to stay the execution. Johnson's attorneys argued in recent appeals that he has had delusions about the devil using his death to bring about the end of the world. Johnson suffered from schizophrenia.

Justice Sonia Sotomayor wrote in a dissent joined by Justices Ketanji Brown Jackson and Elena Kagan that "executing a prisoner who has lost his sanity has, for centuries, been branded inhumane."

"The Court today paves the way to execute a man with documented mental illness before any court meaningfully investigates his competency to be executed," the dissent continued. "There is no moral victory in executing someone who believes Satan is killing him to bring about the end of the world. Reasonable jurists have already disagreed on Johnson's entitlement to habeas relief. He deserves a hearing where a court can finally determine whether his execution violates the Eighth Amendment. Instead, this court rushes to finality, bypassing fundamental procedural and substantive protections."

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, adding that "he knew exactly what he was doing."

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Casey disappeared from her hometown of Valley Park on July 26, 2002, prompting a frantic search until her body was found.

The girl's mother had been best friends with Johnson's older sister during her childhood and even helped babysit him. Following a barbecue the night before the killing, Casey's family allowed Johnson to sleep on a couch in their home.

The following morning, Johnson lured the girl to the abandoned glass factory, even carrying her on his shoulders on the walk, according to court documents. Casey screamed and attempted to break free after Johnson tried to sexually assault her. He then beat her to death with a brick and a large rock before washing off in the nearby Meramec River. Johnson O that same day.

"It was more violent and brutal than any case I've ever seen," former St. Louis County homicide investigator Paul Neske said. Neske had questioned Johnson at length on the day of the murder and witnessed his execution.

Casey's body was found in a pit, buried under rocks and debris, less than a mile from her home.

Defense lawyers at Johnson's trial presented testimony showing their client had stopped taking his schizophrenia medication and was acting strange in the days before the killing. Johnson was an ex-convict who had been released from a state psychiatric facility six months before the murder.

The Missouri Supreme Court denied an appeal in June attempting to block the execution over claims that Johnson's schizophrenia prevented him from understanding the connection between his crime and the punishment. A three-judge federal appeals court panel last week temporarily halted execution plans, but the full 8th U.S. Circuit Court of Appeals reinstated it.

Johnson's attorneys then filed appeals to the Supreme Court regarding his competency to be executed.

Gov. Mike Parson denied a request on Monday from Johnson's attorneys to reduce his sentence to life in prison. The clemency petition argued that Casey's father, Ernie Williamson, opposed the death penalty.

But Casey's great aunt, Della Steele, wrote to the governor urging the execution be carried out to "send the message that it is not okay to terrorize and murder a child." She said grief from Casey's death led to destructive effects among family members.

"He did something horrible," Steele said in a recent interview. "He took a life away from a completely innocent child, and there have to be consequences for that."

Johnson's execution was the 16th carried out so far this year in the U.S., including three others 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."

The Associated Press contributed to this report.