WASHINGTON, United States (AFP) — The United States is today set to carry out its first execution of 2019, of a man convicted of murdering a police officer 30 years ago during an attempted robbery.

Robert Mitchell Jennings, 61, gunned down Elston Howard in a Houston adult bookstore on July 19, 1988, according to a court filing and local media.

Barring a last-minute stay, he will be put to death by lethal injection at a prison in Huntsville, Texas.

Howard, 24, was in the middle of arresting an employee of the bookstore for illegally showing pornographic movies when Jennings entered, having just robbed a neighbouring adult movie theatre.

Jennings walked up to Howard and shot him twice. After the officer fell, Jennings fired again before fleeing to his getaway vehicle, where he told an accomplice he had shot a “security guard.”

The accomplice told Jennings to get out of the car and shot him in the hand when he refused. Wounded, Jennings made his way to a hospital and was arrested.

He has launched multiple appeals since sentence was passed in 1989, his supporters highlighting his difficult upbringing and intellectual limitations in their pleas for clemency.

This week, a court rejected his latest appeal, which focused on deficiencies in his legal defence in earlier cases.

His lawyers have sought a stay from the Supreme Court, which will decide on the matter today.

The US carried out 25 executions in 2018, 13 of them in the state of Texas alone.