WASHINGTON (AFP) - A brain-damaged 74-year-old man who killed a police officer was executed on Tuesday in Missouri after his final Supreme Court appeal was rejected, prison authorities said. Cecil Clayton was pronounced dead by lethal injection at 9.21pm, said Mr Mike O'Connell, spokesman for the Missouri prison service.

Clayton had suffered an injury in a workplace accident in 1972 and had to undergo brain surgery. But the court rejected his argument that he should not be executed because of his mental incapacity.

His lawyers Elizabeth Carlyle and Pete Carter argued that his behaviour changed after the surgery that saw a fifth of the frontal lobe of his brain removed because a piece of wood had pierced through his skull. Then aged 32, he suffered hallucinations, bouts of depression and schizophrenia, and violent tantrums, they said.

It was during one of those episodes that a sheriff's deputy was shot and killed when responding to a call about domestic violence in 1996.

On Jan 2, clinical and forensic psychologist Daniel Foster, who spent time with Clayton, supported the defence argument.

"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," Dr Foster wrote.