Warning: This story contains details about methods of execution that some readers may find distressing.

His last words were “I love you”, followed by a Muslim prayer. Then Charles Brooks Jr – a convicted murderer – looked away from his girlfriend and felt death creep in.

He was lying on a white stretcher, dressed head-to-toe in typical 80s fashion, including gold pants and a shirt with all the buttons open. He had an intravenous line in one arm and doctors hovered nearby. The man could have been a hospital patient.

Instead, his final moments were spent in the death chamber at a Texas prison. It was 1982 and this was the first time the lethal injection had been used to kill a criminal in the United States.

Before this pioneering moment, the nation’s favourite mode of execution was the electric chair, which is today widely regarded as torture. It was so violent, sometimes the victim’s eyeballs would pop out and rest on their cheeks. It regularly set hair on fire, leading guards to stash extinguishers nearby, just in case.

The lethal injection was hailed as kinder and more technologically advanced, with no blood and no screaming. One witness to Brooks' death said that he simply yawned and heaved his stomach. Minutes later, a doctor said “I pronounce this man dead.”

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To this day, the method is the first choice in every US state where capital punishment is legal. But it might not be quite as peaceful as it looks. The problem is, no one actually checked. There was no research or testing of any kind.