Smith’s execution by “nitrogen hypoxia” took around 22 minutes, according to media witnesses, who were led into a viewing room at the William C Holman correctional facility in Atmore shortly before 8 pm local time.

He used sign language to say “I love you” to witnesses in the viewing room, and in his final statement he said: “Tonight, Alabama caused humanity to take a step backward.”

Volker Turk, the UN high commissioner for Human Rights, said on Friday: “I deeply regret the execution of Kenneth Eugene Smith in Alabama despite serious concerns this novel and untested method of suffocation by nitrogen gas may amount to torture, or cruel, inhuman or degrading treatment.”

“According to leading experts, this method is a particularly cruel and unusual punishment, in addition to the fact that the inmate was already subjected to a failed execution attempt in November 2022,” it said in a statement.

Bryan Stevenson, a well-known lawyer who has fought against the death penalty and founded the non-profit Equal Justice Initiative, also condemned Smith’s execution.

Kay Ivey, Alabama’s Republican governor, said the execution was “lawfully carried out by nitrogen hypoxia, the method previously requested by Mr Smith as an alternative to lethal injection”.

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