JEFFERSON CITY, Mo. (AP) — Missouri Gov. Mike Parson said Monday that the state will proceed with this week’s planned execution of a man who abducted and killed a 6-year-old girl nearly two decades ago, though the man’s attorneys are still pressing claims he is mentally incompetent.

Johnny Johnson, 45, is scheduled to receive a lethal injection Tuesday evening for the July 26, 2022, fatal beating of Casey Williamson in her St. Louis County hometown of Valley Park.

Johnson’s attorneys have asked the U.S. Supreme Court to halt the execution. They also asked that Parson grant clemency — reducing Johnson’s sentence to life in prison — while asserting that Johnson’s mental illness has left him incapable of understanding the connection between his actions and his execution.

Parson denied the clemency request. He noted that a variety of courts — including the original trial judge and 8th U.S. Circuit Court of Appeals just days ago — have thus far rejected Johnson’s incompetency claims.

“Johnny Johnson’s crime is one of the most horrific murders that has come across my desk,” Parson said in a statement. “Casey was an innocent young girl who bravely fought Johnson until he took her life.”

Parson, a former sheriff, said he received “countless letters in the last few weeks seeking justice for Casey” and hopes that carrying out the execution “may provide some closure for Casey’s loved ones.”

In a clemency petition, Johnson’s attorneys said that Casey’s father, Ernie Williamson, opposes the death penalty.

But other relatives urged the governor to let the execution go forward.

Casey’s great aunt, Della Steele, sent a message to Parson emphasizing that Johnson could have turned back at any point but instead chose to abduct, assault and kill Casey and then literally covered up his crime. The grief from Casey’s death led to destructive events in the lives of other family members, she wrote in the message, which she shared with The Associated Press.

“Please stand strong beside Casey,” Steele wrote. “Remove this threat from our presence. Send the message that it is not okay to terrorize and murder a child.”

Casey’s mother had been best friends in childhood with Johnson’s older sister and had even helped babysit him. When Johnson showed up at a barbecue, Casey’s family let him sleep on a couch in the same house where they were spending the night.

In the morning, however, Johnson lured the girl out of the home to an abandoned glass factory, even carrying her on his shoulders on the walk. 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 to the crimes that same day.

After a search involving first responders and volunteers, Casey’s body was found in a pit not too far from her home, buried beneath rocks and debris.

At Johnson’s trial, defense lawyers presented testimony showing that he had stopped taking his schizophrenia medication and was acting strangely in the days before the slaying.

Johnson would be the fourth person executed this year in Missouri, and the 16th nationally.