Alabama used nitrogen gas Thursday to execute a man convicted of killing three people in back-to-back workplace shootings.

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.

This is the fifth inmate executed in the United States within one week. This is also the greatest use of capital punishment the country witnessed in two decades