'Tantamount to torture': US prisoner executed by nitrogen gas

An American prisoner has been executed by inhaling nitrogen gas – only the second inmate in the United States’ history to be killed this way.

Alan Eugene Miller shook and trembled for several minutes as he was put to death in Alabama on Thursday evening.

The 59-year-old has been on death row since 2000 for shooting dead three male colleagues - Lee Holdbrooks, Christopher Scott Yancy and Terry Jarvis - a year earlier, believing they were spreading rumours about him at work.

Miller had always sought to be killed by nitrogen gas and had even undertaken legal proceedings to challenge the state's nitrogen hypoxia protocol to achieve that.

“I didn’t do anything to be in here,” Miller said in his final words, which were muffled by a mask that covered his face from forehead to chin.

Critics have called the method 'tantamount to torture'.

The mask was fitted - and re-adjusted - for about 15 minutes as the nitrogen gas flowed.

Miller shook and trembled on a gurney for about two minutes while his body also pulled against restraints.

The shaking and trembling was followed by about six minutes of periodic gulping breaths before Miller became still.

Alabama Department of Corrections Commissioner John Hamm said all went to plan.

“There’s going to be involuntary body movements as the body is depleted of oxygen. That is nothing we did not expect,” he said.

“Everything went according to plan and according to our protocol, so it went just as we had planned.”

Two years ago, Alabama officials tried to put Miller to death by lethal injection but failed because they could not access his veins within the required time limit.

The first execution by nitrogen hypoxia happened earlier this year when Alabama executed Kenneth Smith.

Witnesses said Smith was shaking and writhing on the gurney for minutes before he died.

The United Nations have “unequivocally condemned” the method, saying it is nothing short of "state-sanctioned torture".

“The use, for the first time in humans and on an experimental basis, of a method of execution that has been shown to cause suffering in animals is simply outrageous,” the UN experts said following Smith's death earlier this year.

The states of Louisiana, Oklahoma and Mississippi have also authorised death by nitrogen hypoxia.

Miller had nine visitors in his final hours and a meal of hamburger steak, baked potato and French fries.

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Alabama Attorney General Steve Marshall said the execution of Miller “went as expected and without incident”.

“Despite misinformation campaigns by political activists, out-of-state lawyers, and biased media, the state proved once again that nitrogen hypoxia is both humane and effective,” he said in a statement.

Five inmates - including Miller - have been put to death in the United States over the past week.

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