Lawyers for cop killer, 74, who was left with an IQ of 71 after part of his brain was removed try to stop his execution scheduled for tomorrow

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Lawyers for a Missouri man who shot dead a sheriff's deputy years after doctors removed part of his brain are asking the U.S. Supreme Court to halt his execution, which is scheduled for tomorrow.

Cecil Clayton, from Purdy, is set to be put to death at the state prison in Bonne Terre at 6pm on Tuesday for the November 1996 shooting of Barry County Sheriff's Deputy Christopher Castetter.

But his attorneys are arguing that mental illness and an intellectual disability, which is the result of a traumatic brain injury, make the 74-year-old ineligible for execution, NBC reported.

Their last-ditch effort to save his life comes after the Missouri Supreme Court refused to halt the execution over the same claims. On Saturday, the court issued the 4-3 ruling that he should die.  

Facing death: Cecil Clayton is scheduled to die in Missouri on Tuesday evening but his lawyers are asking the U.S. Supreme Court to stop it. Years before he murdered a cop, he had part of his brain removed, they say

Missing: A brain scan shared by his lawyers show a significant section of his brain is missing

His lawyers have explained that in 1972, 24 years before he killed the officer, Clayton was using a sawmill when a board splintered and a shard went into his head above his right eye. 

Doctors were forced to remove 20 per cent of his frontal lobe, the area of the brain that controls decision making and emotions, the St Louis Post-Dispatch reported.

A scan shared by his attorney Elizabeth Unger Carlyle shows a large section missing from his brain.

Following the accident and before the arrest, Clayton frequently went to mental health facilities, spending up to 15 months at a time under psychiatric care.

Then the evening before Thanksgiving in 1996, Deputy Castetter responded to a call about a suspicious vehicle parked outside Clayton's girlfriend's mother's home.

Earlier that night, Clayton and his girlfriend had been arguing, and prosecutors said he then took a loaded gun to her mother's house.

Clayton walked up to the deputy's patrol car window and shot him in the head at point-blank range. Castetter, a 29-year-old married father-of-three, died in the hospital later that day.

He was convicted of the murder in October 1997 and sentenced to death later that year.

His attorneys have argued that he was mentally incompetent to stand trial but despite their appeals, state and federal appeals courts have upheld the conviction and the death penalty.

His lawyers are seeking a competency hearing to determine whether he understands his death penalty.

Killed: Barry County Sheriff's Deputy Christopher Castetter, pictured left, was shot dead by Clayton, pictured right in an old mug shot, as he responded to a call about a suspicious vehicle in 1996

At the weekend, when the Missouri Supreme Court issued its decision, three of the seven judges agreed that he is entitled to a competency hearing.

Mental health professionals who have examined him agree that he is mentally impaired and has mental health issues, including dementia.

His most recent IQ test found he had an IQ of just 71 - 29 points below the average.

'He is not simply incompetent legally; he would be unable to care for himself or manage basic self-care, were he not in a structured environment that takes care of him,' according to a doctor who examined him, the Kansas City Star reported.

'He can shower, groom, eat, walk; it is his comprehension, judgment, memory, limited intelligence and social deficits that plague him.' 

But there is no agreement on how much he understands his death penalty or his other options.

'If you can prove mental retardation, you can get exempted, but mental illness alone is not an exemption to the death penalty,' Richard Dieter, executive director of the Death Penalty Information Center, told the New York Times. 

In Missouri, the director of the Department of Corrections is the only one with the authority to order a competency hearing, and last year, he called on a doctor to examine Clayton.

Dr. James Reynolds of the Missouri Department of Mental Health concluded that Clayton is mentally ill, but could not be certain that he doesn't understand his death sentence.

Execution: He is scheduled to be put to death at the state prison in Bonne Terre, pictured, next Tuesday

Death: His family and lawyers are arguing that he is child-like and does not understand the significance of his death penalty. They are calling for his execution at Bonne Terre (pictured) to be stopped

But Clayton's lawyers say, while he may have conversations in which he understands he is about to be executed, he will later forget that conversation ever happened.

His family have also called for a fair hearing on the issue. 

'He is brain-damaged, and talking with him is like talking to a child,' Clayton's daughter, Jena Clayton, said during his clemency petition, the Kansas City Star reported.

'I do not believe we are the kind of country that executes the disabled.' 

Clayton, who is the state's oldest death row inmate, will be put to death between 6pm on March 17 and 5.59pm on March 18. 

He will become the second person put to death in the state in 2015, after a record 10 executions in Missouri in 2014. Last month, Walter Storey was put to death for slitting his neighbor's throat 25 years ago.