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 lingering brain-damage effects from a 1972 sawmill accident.

Here's a look at the case:

___

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 report of a suspicious vehicle near Cassville on the night before Thanksgiving 1996 when he was shot in the forehead while he was in his car. The vehicle was found against a tree with its engine running fast and wheels spinning. Castetter died at a hospital the next day.

___

INMATE'S CLAIMS

Clayton's attorneys argue that a 1972 sawmill accident in which a piece of wood shot through Clayton's skull forced surgeons to remove about 8 percent of his brain, including one-fifth of the frontal lobe portion governing such things as impulse control and judgment. Combined with his reported IQ of 71, they say psychiatric evaluations over the past decade have concluded that Clayton doesn't understand the significance of his scheduled execution or the reasons for it, making him ineligible for being put to death under state and federal law.

Given the number of mental health experts who consistently have 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 turned prone to alcohol abuse and violent outbursts.

___

LEGAL SETBACKS

The Missouri Supreme Court, in a 4-3 ruling, declined to intervene Saturday. The court's majority concluded there's no evidence Clayton — despite his brain injury and advanced age — 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."

___

SIMILAR CASES

Clayton's claims of mental incompetence mirror those of Ricky Ray Rector, who was executed in 1992 in Arkansas for the shooting death 11 years earlier of a police officer. Rector was 40 when he was put to death, having failed to sway then-Gov. Bill Clinton that he was left brain-damaged by a self-inflicted bullet wound prior to his arrest.

___

MISSOURI EXECUTIONS

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