A convicted murderer has been executed in Missouri after both the state’s governor and the US Supreme Court declined to grant clemency sought by Pope Francis on the grounds that the man was intellectually disabled.

Ernest Johnson, 61, was found guilty of killing three convenience store employees in 1994.

He was injected with a lethal dose of pentobarbital, a powerful barbiturate, in the state’s execution chamber in the city of Bonne Terre on Tuesday evening.

He was pronounced dead at 6.11pm (11.11pm GMT), the Missouri Department of Corrections said in a statement.

But Johnson’s lawyers said there was overwhelming evidence of his intellectual disability.

They asked the US Supreme Court on Tuesday to halt the execution because executing intellectually disabled people breaches a constitutional ban on “cruel and unusual punishments”.

The conservative-majority Supreme Court rarely blocks executions, and denied Johnson’s petition in a brief unsigned
order late Tuesday afternoon.

The Missouri Supreme Court had rejected Johnson’s intellectual disability claim in August.

Johnson was born with foetal alcohol syndrome disorder, scored low on IQ tests throughout his life and had the “daily
living skills” of a four-year-old child, his lawyers said.

The pope and former Missouri governor Bob Holden, a Democrat, had asked that Johnson’s life be spared.

Current Governor Mike Parson denied clemency on Monday for what his office called a “brutal murder”.

“Mr Johnson’s claim that he is not competent to be executed has been reviewed and rejected by a jury and the courts six different times, including a unanimous decision by the Missouri Supreme Court,” Mr Parson’s office said in a statement.

A jury found Johnson guilty of bludgeoning to death Mary Bratcher, Mabel Scrubbs and Fred Jones using a hammer, a screwdriver and a gun.