Missouri's oldest death row inmate was executed Tuesday for the 1996 shooting death of a sheriff's deputy, after the U.S. Supreme Court and the governor declined to spare the 74-year-old whose attorneys said he had a diminished mental capacity because of a sawmill accident decades ago.

Cecil Clayton was put to death by lethal injection after Gov. Jay Nixon denied a clemency request and the nation's high court turned aside appeals claiming Clayton was mentally incompetent. The Missouri Supreme Court, in a 4-3 ruling, already had declined to intervene, with the court's majority concluding last weekend there was no evidence Clayton -- despite his brain injury -- wasn't capable of understanding his circumstances. The U.S. Supreme Court was also divided, with four judges saying they would have granted a stay.

Mike O'Connell, spokesman for the Missouri Department of Corrections, said in a statement that Clayton was executed at 9:13 p.m. and pronounced dead at 9:21 p.m.

As the execution began, Clayton appeared to breathe heavily for about a minute, and the sheet over his right leg quivered slightly. His mouth slowly went agape, but there was no other movement before he was pronounced dead.

The claim of Clayton's diminished mental capacity stemmed from a 1972 sawmill accident that his attorneys argued cost him about 8 percent of his brain, including one-fifth of the frontal lobe portion governing impulse control and judgment.

Combined with his reported IQ of 71 and reading skills of a fourth-grader, Clayton's attorneys insisted psychiatric evaluations over the past decade concluded that the inmate didn't understand the significance of his scheduled execution or the reasons for it, making him ineligible to be put to death under state and federal law.

In their 11th-hour appeals, Clayton's attorneys argued that his deteriorating mental health left him convinced his conviction was a plot against him and that God would rescue him from a death sentence at the last minute, "after which time he will travel the country playing the piano and preaching the gospel."

Clayton was convicted of gunning down Christopher Castetter, a sheriff's deputy in rural southwest Missouri's Barry County. Castetter was 29 and a father of three when he went to a home near Cassville on Thanksgiving Eve 1996 to check on a suspicious vehicle report. Authorities said Clayton shot Castetter once in the forehead while the deputy was in his car, which was found against a tree, its engine racing and wheels spinning.

Clayton's brother had testified that the sawmill accident led to Clayton's breakup from his wife, alcohol abuse and violent outbursts.

The lethal injection, Clayton's attorneys said, was "sure or very likely to cause excruciating or tortuous pain and needless suffering" in light of his dementia.

"If Missouri proceeds with its scheduled execution of Mr. Clayton, it will be conducting an unregulated experiment on a human subject, as there are no studies that support (the prison system's) use of Missouri's lethal injection protocol on an individual suffering from severe brain damage," the appeals on Clayton's behalf argued.

Clayton's claims of mental incompetence mirrored 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 -- campaigning for what at the time later became his first presidential term -- that he was left brain-damaged by a self-inflicted bullet wound prior to his arrest.

The execution was Missouri's second this year after the state's record 10 in 2014. It was also the first Missouri carried out in the evening after decades of having them just after midnight.