The state of Alabama on Thursday executed inmate Alan Eugene Miller with nitrogen gas, making him the second to die by the controversial capital punishment method.

Miller, 59, was sentenced to death in 2000 for fatally shooting three of his former co-workers in 1999.

A previous attempt to execute Miller in 2022 was called off when officials were unable to access his veins. Thursday’s execution came after a lawsuit in which Miller argued the nitrogen gas could cause him undue suffering, violating his Eighth Amendment protections against cruel and unusual punishment.

Alabama Attorney General Steve Marshall said the settlement was proof the method – which had been used for the first time to execute Kenneth Smith earlier this year – was constitutional.

Defenders of the method – nitrogen hypoxia – argue it causes the person to lose consciousness quickly as they breathe in 100% nitrogen instead of oxygen. However, doctors have said it’s next to impossible to determine when someone loses consciousness during the execution.

Critics have said dying by nitrogen hypoxia is akin to being tortured to death since it can cause excessive pain while the person suffocates.

Witnesses to the execution reported him shaking on a gurney for several minutes before he died. Witnesses to Smith’s execution reported similar observations.

“Everything went according to plan and according to our protocol,” Alabama Department of Corrections Commissioner John Hamm said to reporters after Miller died.

Louisiana, Oklahoma and Mississippi have also authorized the use of nitrogen gas in executions, but have not yet used it.