Alan Miller was seen shaking and trembling before gasping for air as he died at a US prison in the state of Alabama.

The 59-year-old, who was strapped to a stretcher, was also seen pulling at his restraints before he stopped moving.

He was only the second person to be executed in the US using the gas.

Miller was convicted of shooting dead three men in the city of Pelham, Alabama, in August 1999.

Officials previously tried to execute him by lethal injection in 2022 but abandoned the attempt after staff failed to find a vein before a deadline for the expiration of his initial death warrant.

In his final words, Miller said: "I didn't do anything to be in here."

He also asked his family and friends to "take care" of someone, but the individual's name was not clear because his voice was muffled by a gas mask covering his face.

The convict was pronounced dead at 6.38pm local time on Thursday.

The execution, the fifth in the US in the space of a week, has sparked renewed debate about the death penalty and whether some methods are inhumane.

Campaigners from US groups such as Death Penalty Action have described the use of nitrogen gas as "horrific and torturous".

Alabama's attorney general Steve Marshall said the execution "went as expected and without incident".

He added: "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."

'Pure evil'

Alabama Corrections Commissioner John Hamm said: "Everything went according to plan and according to our protocol."

However, Lauren Gill, a journalist for local politics magazine Bolts, wrote on X: "I was a witness for Alabama's execution of Alan Miller by nitrogen gas tonight. Again, it did not go as state officials promised.

"Miller visibly struggled for roughly two minutes, shaking and pulling at his restraints. He then spent the next five to six minutes intermittently gasping for air."

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Some experts, including from the American Veterinary Medical Association, consider nitrogen gas to be "unacceptable" as a method of euthanasia for most types of animals due to the distress it can cause.

Miller, a former delivery truck driver, shot dead two colleagues at Ferguson Enterprises - Lee Holdbrooks, 32, and Christopher Scott Yancy, 28 - during his killing spree.

He then drove five miles to his previous workplace, Post Airgas, where he killed Terry Jarvis, 39.

All three were shot several times.

A court heard he was paranoid and believed his co-workers had been gossiping about him.

Following the execution, Alabama Governor Kay Ivey described Miller as "pure evil" and said justice had been "finally served".

Family members of the victims did not witness the execution, state officials said, and did not issue a statement afterwards.