The US state of Alabama has executed convicted murderer Kenneth Smith using nitrogen gas for the first time.
The state has called its new protocol "the most painless and humane method of execution known to man".
Smith, convicted of a 1988 murder-for-hire, was a rare prisoner who had already survived one execution attempt. In November 2022, Alabama officials aborted his execution by lethal injection after struggling for hours to insert an intravenous line's needle in his body.
The execution began at 7:53 pm (0153 GMT Friday) and Smith was declared dead at 8:25 pm (0225 GMT), prison officials said.
United Nations human rights experts and lawyers for Smith had sought to prevent it, saying the method was risky, experimental and could lead to a torturous death or non-fatal injury.
The US Supreme Court's conservative majority rejected Smith's final attempt to have his execution delayed to allow his legal challenge to continue on Thursday evening.
The court did not explain its reasoning in denying Smith's appeal, but the three liberal justices offered written dissents.
Lawmakers in Oklahoma and Mississippi have also approved similar nitrogen-asphyxiation execution protocols in recent years, but have yet to put them into practice.