Alabama conducted the United States' second execution with the use of nitrogen gas on Thursday evening, finally killing Alan Eugene Miller for the fatal 1999 workplace shooting of three people.

Alabama executed a condemned man Thursday using nitrogen gas in only the second instance in the United States of the method's use, which drew criticism from human rights groups.

Alabama executed convicted murderer Alan Miller on Thursday in the second-ever nitrogen-asphyxiation execution since the state pioneered the method which it says is less painful than lethal injections but human rights experts say may amount to torture.