Bonne Terre, Missouri - The state of Missouri is planning to move forward with an execution of Johnny Johnson on Tuesday despite legal challenges and pleas for clemency.

Johnson is scheduled to receive a lethal injection on Tuesday evening at the Eastern Reception, Diagnostic and Correctional Center in Bonne Terre after the US Court of Appeals for the Eighth Circuit overruled a temporary stay.

The 45-year-old was convicted of murdering six-year-old Casey Williamson in 2002 in St. Louis County after attempting to rape her.

Johnson's legal team has argued that he was suffering severe psychosis at the time of his crime.

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Lawyers also pointed out Johnson has suffered mental health problems since he was a child, including schizophrenia, PTSD, and a brain disorder that caused developmental delays. He was also the victim of physical and sexual abuse as a minor.

Petitioners on behalf of Johnson have noted in their clemency request that he does not understand the reason for his execution, believing that "Satan is using the State of Missouri to bring about the end of the world."

Missouri's Republican Governor Mike Parson has given no sign that he will heed calls for clemency.

"Casey was an innocent young girl who bravely fought Johnson until he took her life," Parson said on Monday. "My office has received countless letters in the last few weeks seeking justice for Casey. Although this won't bring her back, we hope that carrying out Johnson's sentence according to the Court's order may provide some closure for Casey's loved ones."

Missouri Representatives Cori Bush and Emanuel Cleaver II are among the leaders who have urged the governor to put a stop to the killing, writing, "Nothing is lost by halting the execution to assess Mr. Johnson's competency, as the Eighth Circuit ruled; in contrast, his life stands to be unlawfully taken."

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"Each of us is more than the worst thing we've ever done. The same is true for Mr. Johnson," they added.

Johnson's lethal injection is set to take place at 6 PM. Unless there is a last-minute intervention, he will be the fourth person to be executed in Missouri so far this year, following Amber McLaughlin in January, Leonard "Raheem" Taylor in February, and Michael Tisius in June.