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.

Alan Eugene Miller has been executed in Alabama, according to Gov. Kay Ivey’s office, making him the second inmate known to die by nitrogen gas, a controversial new capital punishment method. Miller was sentenced to death in 2000 for the 1999 killings of three men.