Alan Eugene Miller was executed Thursday evening in Alabama, state officials said, making him the second inmate known to die by nitrogen hypoxia, a controversial method critics say is tantamount to torture.

After carrying out the country’s first nitrogen execution in January with what some witnesses described as distressing results, Alabama is trying again.

Alabama is set to carry out the nation’s second execution ever using nitrogen gas after becoming the first state to use the new procedure in January. Alan Miller is set to die by the process on Thursday,

Alan Eugene Miller, 59, is scheduled to be executed with nitrogen gas at a south Alabama prison two years after the state unsuccessfully attempted to execute him by lethal injection. Miller was convicted of killing three men — Lee Holdbrooks, Christopher Scott Yancy and Terry Jarvis — in back-to-back workplace shootings in 1999.

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.

Carey Dale Grayson, 49, is set to die on Nov. 21. He would be the third Alabama inmate to die this year using nitrogen gas.

In Montgomery, Alabama, a federal judge heard testimonies on Tuesday about the nation's first two nitrogen gas executions after lawyers for death row inmate Carey Dale Grayson asked for an injunction to halt his execution.