John Middleton was executed Wednesday evening by the state of Missouri. He was the sixth person executed in the state this year and the 25th put to death nationwide. Middleton, 54, a former meth dealer, was convicted and sentenced to death in 1997 for three murders in 1995.

The US Supreme Court refused to halt his execution despite significant evidence of his mental incompetency. The Missouri Supreme Court and a federal appeals court also denied his appeals, while Missouri Governor Jay Nixon, a Democrat, declined to grant him clemency.

Middleton was injected with a dose of pentobarbital at 6:58 p.m. local time and pronounced dead at 7:06 p.m.

Middleton’s lethal injection had originally been scheduled for Tuesday. But less than two hours before the 12:01 a.m. deadline for his execution, US District Court Judge Catherine Perry halted it, arguing that his lawyers had “made a significant threshold showing he is incompetent to be executed,” and that he should be granted a legal hearing to evaluate his sanity.

A psychologist who examined Middleton stated in an affidavit that he lacked “a rational understanding of the reason for the execution and is therefore not competent to be executed due to a diagnosis of delusional disorder, a psychotic mental illness.”

Judge Perry stated that the affidavits provided “from other inmates and from the counsel who have dealt with him indicate that his mental state has deteriorated over the 17 years he has been incarcerated.” She continued: “The inmates indicate that he frequently talks to people who are not there, and tells stories that could not have had any basis in reality.”

Missouri has executed one person every month this year but one. Only Florida and Texas, which have each executed seven people, have carried out more executions in 2014. Last month, Missouri executed John Winfield, one of three lethal injections carried out in a 24-hour period in the US.

The three condemned inmates—John Winfield in Missouri, Marcus Wellons in Georgia, and John Henry in Florida—were the first to be put to death in the US since the April 29 state killing of Clayton Lockett in Oklahoma.

Lockett, 38, suffered for 43 minutes during his lethal injection procedure before dying of cardiac arrest. Some 51 minutes after being strapped to the execution gurney, Lockett was injected in his groin with a three-drug cocktail of midazolam, veruconium bromide and potassium chloride.

Within minutes of the injection, he began twitching violently and the curtains were drawn to obscure the view of witnesses. Prison authorities then reportedly called off the execution. Some 43 minutes later reporters were told that Lockett had died of a massive heart attack.

The difficulties in procuring deadly chemicals and the controversy over the lethal injection procedure have some states seeking alternative methods for sending prisoners to their deaths and keeping the state killing machine in operation.

While the death penalty still exists in 32 of the 50 US states, executions have been carried out in only 10 of these states in recent years.