Missouri's oldest death row inmate, scheduled to be executed Tuesday by injection for the 1996 shooting death of a sheriff's deputy, is asking the U.S. Supreme Court and the state's governor to spare his life. Attorneys for Cecil Clayton, 74, argue in last-minute appeals and a clemency request that Clayton has dementia and brain damage from a 1972 sawmill accident.

Here's a look at the case:

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THE CRIME

Clayton was convicted of gunning down Christopher Castetter, a sheriff's deputy in rural southwest Missouri's Barry County. Castetter, then 29 and a father of three, was investigating a suspicious vehicle near Cassville on the night before Thanksgiving 1996 when he was shot in the forehead while he was in his car. His vehicle was found against a tree with the engine running fast and wheels spinning. Castetter died at a hospital the next day.

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INMATE'S CLAIMS

Clayton's attorneys argue that their client suffers the lingering effects of a 1972 sawmill accident in which a piece of wood shot through his skull. Surgeons removed about 8 percent of Clayton's brain, including one-fifth of the frontal lobe that governs impulse control and judgment. They say Clayton has an IQ of 71 and that psychiatric evaluations indicate he doesn't understand the significance of his scheduled execution or the reasons for it, making him ineligible for execution under state and federal law.

Given the number of mental health experts who have consistently found Clayton to be intellectually incompetent, "normally you have someone say he's malingering or cheating on the test or making this up, and you just don't have any of that here," Cynthia Short, a Clayton attorney, said Monday. Clayton's brother testified during his trial that his sibling broke up with his wife after the sawmill accident and became prone to alcohol abuse and violent outbursts.

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LEGAL SETBACKS

The Missouri Supreme Court, in a 4-3 ruling, declined to intervene Saturday. The court's majority concluded there's no evidence that Clayton — despite his brain injury — isn't capable of understanding his circumstances. The dissenting opinion countered that Clayton's attorneys "presented reasonable grounds to believe his overall mental condition has deteriorated and he is intellectually disabled."

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SIMILAR CASES

Clayton's claims of mental incompetence mirror those of Ricky Ray Rector, who was executed in 1992 in Arkansas for fatally shooting a police officer. His attorneys failed to sway then-Gov. Bill Clinton that he had been left brain damaged by a self-inflicted bullet wound prior to his arrest.

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MISSOURI EXECUTIONS

If carried out, Clayton's execution would be Missouri's second this year. The state executed a record 10 people in 2014. Clayton's execution also is the first to be scheduled in Missouri for 6 p.m. Tuesday after decades of having lethal injections set to begin at 12:01 a.m. Wednesday.