Washington (AFP) — A death row inmate facing execution Tuesday night won a last-minute stay on the grounds he might be mentally incompetent.

But the state of Missouri, where John Middleton, 54, had been scheduled to be put to death for killing three people in 1995, filed an appeal, meaning he could still be executed some time Wednesday.

Federal Judge Catherine Perry granted the stay less than an hour before Middleton was scheduled to die by lethal injection in the city of Bonne Terre, a minute after midnight Tuesday, said Missouri penal system spokesman Mike O’Connell.

The U.S. Supreme Court earlier had rejected an appeal from the inmate’s lawyers and given the green light for the execution to proceed.

But Perry ruled that Middleton was entitled to a stay. “He has shown a significant possibility to win on the merits, in that he has made a threshold showing that he is incompetent to be executed,” she wrote.

Middleton was a methamphetamine user with a history of psychiatric problems, according to the appeal filed with the Supreme Court.

The state of Missouri appealed Perry’s ruling, and the 8th US Circuit Court of Appeals said it will issue a ruling Wednesday morning. That leaves open the possibility of his being executed as late as midnight Wednesday, said O’Connell.

Middleton was sentenced to death in 1997 for the killing of two men on June 10, 1995 and another two weeks later. Prosecutors say Middleton feared they might tell police he was a meth dealer.

If it takes place it will be the sixth execution so far this year in Missouri. Only Florida and Texas, with seven each, have put more people to death in 2014.

AFP Photo / Brendan Smialowski

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