It marked the first time that a new execution method has been used in the United States since lethal injection, now the most commonly used method, was introduced in 1982. After he had a chance to make a final statement, the warden, from another room, was to activate the nitrogen gas. And experts appointed by the U.N. Human Rights Council cautioned they believe the execution method could violate the prohibition on torture. Three states — Alabama, Mississippi and Oklahoma — have authorized nitrogen hypoxia as an execution method, but no state had attempted to use the untested method until now. Smith’s attorneys had raised concerns that he could choke to death on his own vomit as the nitrogen gas flows.