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

The planned execution of a 45-year-old Missouri man with schizophrenia is back on after an appellate court reversed course on Saturday.

Johnny Johnson is scheduled to receive a lethal injection on Tuesday at the state prison in Bonne Terre for killing 6-year-old Casey Williamson after trying to sexually assault her in 2002.

With questions swirling about his mental competency, the execution was halted last Tuesday by a divided three-judge panel of the 8th US Circuit Court. But after the Missouri Attorney General’s Office asked that the full court reconsider, that decision was reversed in a 7-3 ruling.

The case will likely end up before the US Supreme Court before the scheduled execution date.

Attorneys for Johnson have claimed his schizophrenia prevents him from understanding the link between his crime and the punishment.

They have also said Johnson has delusions about the devil using his death to bring about the end of the world.

The Missouri Supreme Court in June declined to halt the execution based on the mental health claim.

The attorney general’s office challenged the credibility of psychiatric evaluations of Johnson and contended medical records indicate he is able to manage his mental illness through medication.

Johnson lured the girl to an abandoned glass 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. He killed her with bricks and rocks, then washed off in the Meramec River.

Johnson confessed to the crimes. Casey’s disappearance set off a frantic search involving first responders and volunteers.

Her body was found in a pit less than 1.6 kilometres from her home, buried beneath rocks and debris.

The execution would be the fourth in Missouri this year.