Kenneth Smith is due to die in Alabama on January 25 for murdering the wife of a church pastor who was deeply in debt and wanted her dead for insurance money.
Smith - who was paid for the murder - spent hours strapped to a gurney in 2022 while officials repeatedly tried and failed to insert an intravenous line to deliver a lethal injection.
Lawyers say the execution attempt left the 58-year-old with post-traumatic stress disorder and he's terrified as the state prepares to try again.
Kenneth Smith is due to die in Alabama on January 25 for murdering the wife of a church pastor. (AP PHOTO)
This time officials plan to asphyxiate Smith by pumping nitrogen gas into a mask. They say the killer-for-hire will be deprived of oxygen, and should die quickly and humanely while strapped down.
Australian-born doctor and assisted dying advocate Philip Nitschke has served as an expert witness in a court challenge seeking to defer the second attempt later in January.
He's told an Alabama court that elected death advocates rejected the mask-and-gas approach more than 20 years ago because it was unreliable.
He says Smith could be horribly maimed without a complete seal between mask and face - something that's very difficult to achieve.
"There is a significant possibility that Mr Smith will be subject to incomplete cerebral hypoxia. A resultant vegetative state with permanent brain damage cannot be excluded," he has told the court.
"There is no information in the (execution) protocol on the design of the mask, or whether it has been made to custom-fit Mr Smith's face ...
"It is difficult to see how an effective air-seal could be initially established, let alone maintained, without Mr Smith's participation and co-operation."
Dr Nitschke is the founder of Exit International - an organisation that educates people interested in elected deaths about how to end their lives reliably and peacefully.
He says the nitrogen isn't the problem but it must be delivered correctly. Exit International advocates a head covering that doesn't require a perfect seal, and relies on a constant flow of nitrogen to displace oxygen.
Before providing his testimony, Dr Nitschke visited the Holman Correctional Facility where Smith is scheduled to die.
He's told AAP the experience of lying on the execution gurney and having a lawyer for the state of Alabama strap the mask to his head left him horrified.
Dr Nitschke visited the Holman Correctional Facility where Smith is scheduled to die. (AP PHOTO)
"The set-up is macabre ... The optics of a man lying there, tied to the horizontal tray in the execution room with this mask strapped to his face is simply dreadful. It is cruel. It should never be allowed to happen."
If Smith's execution proceeds, capital punishment experts say it will be the world's first judicial execution by asphyxiation with an inert gas.
Smith's lawyers have argued the untested gassing protocol likely violates the US Constitution's ban on cruel and unusual punishments, and they've argued a second bid to kill him by any method is unconstitutional.
Last week, four UN human rights experts urged authorities to halt the planned execution, saying the method could cause grave suffering and a painful death that would likely breach a treaty the US has signed banning torture and other cruel, inhuman or degrading punishments.
The federal judge hearing the challenge is still considering whether to grant a temporary injunction so Smith's lawyers have time to challenge the constitutionality of the mask-and-gas approach in court.
Alabama officials have generally declined to comment with legal proceedings continuing.
Smith was one of two men who were each paid $1000 to kill Elizabeth Sennett in 1988. The other man was executed over a decade ago.
Church of Christ pastor Charles Sennett killed himself when he realised police were closing in on his involvement in the stabbing murder.
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