Alabama used nitrogen gas Thursday to execute a man convicted of killing three people in back-to-back workplace shootings, the second time the method that has generated debate about its humaneness has been used in the country.

Alan Eugene Miller was convicted of killing three men in 1999 and the state had previously attempted to execute him by lethal injection in 2022.

Alan Eugene Miller, 59, is scheduled to be executed with nitrogen gas at a south Alabama prison. Miller was convicted of killing three men — Lee Holdbrooks, Christopher Scott Yancy and Terry Jarvis — in back-to-back workplace shootings in 1999.