Kenneth Smith: Alabama murderer to become first inmate in US executed with 'painless' nitrogen gas

A prisoner in Alabama is set to become the first inmate in America to be put to death with nitrogen after the US Supreme Court declined to halt his execution.

Convicted murderer Kenneth Smith is set to be executed on Thursday after his legal bid to have the method declared unconstitutional failed.

Smith's survival following a botched lethal injection in 2022 helped prompt a review of the state's death penalty procedures.

The Supreme Court justices denied Smith's request to stay his execution and declined to hear his legal challenge contending that a second execution attempt by Alabama - after the first failure caused him severe trauma - would violate the US Constitution's 8th Amendment protections against cruel and unusual punishment.

Smith is separately contesting the legality of Alabama's nitrogen gas protocol.

The 58-year-old's lawyers have urged the US Supreme Court to intervene, calling Alabama's nitrogen gas protocol "recently released and untested" and "a novel method of execution that has never been attempted by any state or the federal government".

Alabama's gassing method - nitrogen hypoxia - is designed to deprive prisoners of oxygen by placing a mask connected to a cylinder of nitrogen over their face.

A state attorney told America's 11th Circuit courts that it will be "the most painless and humane method of execution known to man".

Smith's botched execution was the third consecutive instance where Alabama officials encountered problems or delays inserting intravenous lines for a scheduled lethal injection, with two of the executions, including Smith's, eventually called off, according to court filings.

Execution tried and failed

State officials repeatedly tried and failed to place the necessary intravenous lines or a central line in his collarbone area before calling off the execution.

His lawyers have characterised the experience as torture and said that it "exposed him to the severe mental anguish of a mock execution".

Read more:Texas man executed for murdering five-year-old girlMan executed for rape and murder of dance studentOpenly transgender woman executed for first time in US history

Smith was convicted of the 1988 killing of Elizabeth Sennett after he and an accomplice were hired as hitmen by her husband Charles, a Christian minister who had taken out a large life insurance policy on his wife.

She was stabbed repeatedly and beaten with a blunt object. Charles Sennett later took his own life.

European ban on execution drugs

Smith's accomplice was also convicted and sentenced to death, and executed in 2010.

States with the death penalty increasingly struggle to get the barbiturates used in lethal injection protocols, in part due to a European ban on pharmaceutical companies selling drugs to be used in executions.

Alabama, Mississippi and Oklahoma have all introduced new gas-based protocols.