JEFFESON CITY, Mo. (AP) — A man who abducted a 6-year-old girl and beat her to death at an abandoned glass factory was scheduled to be executed Tuesday in Missouri, as his attorneys pressed claims that he is mentally incompetent.

Johnny Johnson, 45, was convicted in the July, 26, 2002, killing of Casey Williamson, whose disappearance set off a frantic search in her hometown of Valley Park, a small suburb of St. Louis.

Casey’s mother had been best friends in childhood with Johnson’s older sister and had even helped babysit him. 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 to the abandoned factory, even carrying her on his shoulders on the walk to the dilapidated site. When he tried to sexually assault her, Casey screamed and tried to break free. According to court documents, 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.

After a search involving first responders and volunteers, Casey’s body was found in a pit less than a mile (kilometer) from her home, buried beneath rocks and debris.

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

In recent appeals, Johnson’s attorneys have said he has delusions about the devil using his death to bring about the end of the world. They also noted he had been placed on suicide watch in prison a couple years ago after claiming to be a vampire.

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. The Missouri Attorney General’s Office successfully challenged the credibility of the psychiatric evaluation and said medical records indicate that Johnson is able to manage his mental illness through medication.

A three-judge federal appeals court panel last week temporary halted the planned execution, but the full 8th U.S. Circuit Court of Appeals reinstated it. Johnson’s attorneys then filed multiple appeals to the U.S. Supreme Court centered around his competency to be executed.

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

“Johnny Johnson’s crime is one of the most horrific murders that has come across my desk,” Parson, a former sheriff, said in a statement.

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 Parson urging him to proceed with the execution to “send the message that it is not okay to terrorize and murder a child.” Steele said in the message that the 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 told The Associated Press.

Steele has led a variety of community efforts to honor Casey. Through years of fundraising, Casey’s family provided $500 scholarships or savings bonds to all 65 students of the senior class of Valley Park High School in 2014, the year Casey would have graduated.

The family also has organized community safety fairs in Casey’s memory, including a July 22 event that drew a couple hundred people. They gave away dozens of child identification kits along with safety tips involving fire, water and bicycles, among other items.

“A lot of kids from Casey’s class were there with their children. It was nice to see, but it definitely makes you think,” Steele said. “They’re pushing their strollers with their couple of kids and you think, `That’s where she should be.’”

The scheduled execution would be the 16th in the U.S. this year. In addition to three previous executions in Missouri, five have been conducted in Texas, four in Florida, two in Oklahoma and one in Alabama. There were 18 executions in six U.S. states last year.

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Associated Press writer Jim Salter contributed to this report from O’Fallon, Missouri.