Authorities in Alabama put Kenneth Eugene Smith to death Thursday using nitrogen hypoxia, marking the first time the method has been used for execution in U.S. history.

Officials in Alabama said Smith was pronounced dead at 8:25 p.m. after breathing from a mask that delivered pure nitrogen gas.

Wednesday, the U.S. Supreme Court denied a request by Smith to block the execution. Also Wednesday, the 11th U.S. Circuit Court of Appeals cleared the way for Alabama to proceed with the execution.

In execution by nitrogen hypoxia, a person is deprived of oxygen and forced to breathe pure nitrogen. Death results when oxygen levels in the blood drop too low.

Alabama, along with Oklahoma and Mississippi, are the first states to authorize execution by the method.

SEE MORE: Alabama inmate to be first prisoner executed with nitrogen gas

Attorneys for Smith argued that the method of his execution “would constitute cruel and unusual punishment.” They warned that the untested method could cause stroke or a vegetative state.

Alabama said Smith had previously requested the execution method as an alternative to lethal injection.

The state first attempted to execute Smith in 2022 via lethal injection, but the procedure was canceled after those administering it could not connect the required intravenous lines to Smith’s veins. Smith’s lawyers said he spent nearly four hours strapped to a gurney during that attempt. 

Smith was convicted in the murder of Elizabeth Sennett in 1988.

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