Kevin Johnson died after an injection of pentobarbital at the state prison in Bonne Terre.

His 19-year-old daughter, Khorry Ramey, had asked a federal court to allow her to watch her father die, but a judge upheld a state law which bars anyone under 21 from witnessing an execution.

However, in a first for modern executions in Missouri, Johnson was not alone when he died.

The 37-year-old had his spiritual adviser, the Reverend Darryl Gray, beside him.

Rev Gray said afterwards: "We read scripture and had a word of prayer.

"He apologised again. He apologised to the victim's family. He apologised to his family.

"He said he was looking forward to seeing his baby brother. And he said he was ready."

Johnson's lawyers did not deny that he killed 43-year-old William McEntee in 2005, but claimed racism played a role in the decision to seek the death penalty, and in the jury's decision to sentence him to die. Johnson is black and his victim was white.

But the courts, including the US Supreme Court and Republican governor Mike Parson, declined to halt the execution.

Mr McEntee, a married father-of-three, was a 20-year veteran of the police department in Kirkwood, a St Louis suburb.

He was among several police officers sent to Johnson's home on July 5, 2005 to serve a warrant for his arrest for an alleged probation violation.

Johnson's 12-year-old brother, who suffered from a congenital heart defect, collapsed and began having a seizure. He later died in hospital.

When the police officer later returned to the neighbourhood to investigate unrelated reports of fireworks being set off, he was attacked by Johnson.

A court filing from the Missouri attorney general's office said the officer was in his car questioning three children when Johnson shot him several times through the open passenger-side window.

According to the court papers, Johnson then walked down the street and told his mother the officer "let my brother die" and "needs to see what it feels like to die".

Though his mother replied, "that's not true", Johnson returned to the police patrol car where he found the officer on his knees and shot him in the back and in the head, killing him.

Johnson later testified at trial that the officer had prevented his mother from helping his brother.

The officer's wife, Mary McEntee, said in a statement after Tuesday's execution that Johnson had acted as "judge, juror and executioner" in killing her husband.

"Bill was killed on his hands and knees in front of strangers, the people he dedicated his life to," she said.