MONTGOMERY, Ala. (AP) — Alabama’s use of nitrogen gas to put people to death needs more study of whether it violates a constitutional ban on cruel and unusual punishment, a federal appeals court decided Monday.

The state first used nitrogen for capital punishment in 2024, and the ruling could upend Alabama’s next scheduled execution on Thursday. The method involves strapping a respirator to the person’s face and replacing breathable air with pure nitrogen, causing death from lack of oxygen.

The three-judge panel on Monday night reversed a judge’s May finding that the nitrogen method does not violate the U.S. Constitution’s ban on cruel and unusual punishment and remanded the case for additional consideration. The ruling came in a lawsuit filed last year by Jeffery Lee, a man on death row who is scheduled to be executed with nitrogen on Thursday at a south Alabama prison.

The panel stopped short of staying Lee’s planned execution. However, the panel asked the judge to consider whether his proposed alternative of a firing squad was feasible.

The U.S. Supreme Court requires a two-prong test for people challenging the constitutionality of an execution method. They must show the method provides a substantial risk of superadded pain and that a feasible alternative method is available. The appeals court said Lee met the first test but sent it back to the trial court to consider the second.

The appeals panel raised concerns about the nitrogen method and how long it might take the subject to lose awareness.

“In our view, the overall suffering described by the district court, which lasts for one to three minutes, presents a substantial risk of serious harm over and above death itself,” the panel wrote. “Counting to 60 or 180 seconds is not a quick exercise, and constitutionally speaking, that timeframe is intolerable given the suffering that would likely take place under Alabama’s nitrogen hypoxia protocol.”

The Alabama Attorney General’s Office did not immediately issue a comment on the decision. The state has maintained the method is constitutional.

Opponents of the method cheered the decision.

“For the first time a court has acknowledged what I and so many others have seen with our own eyes. Nitrogen executions are a unique form of horror,” said the Rev. Jeff Hood, who was the spiritual adviser at two nitrogen executions.

Nitrogen has been used in eight executions nationally — seven times in Alabama and once in Louisiana. Lee’s attorneys argued it causes excessive suffering. Alabama’s last nitrogen execution took more than 30 minutes to complete.

Lee was convicted of two counts of capital murder for killing Jimmy Ellis and Elaine Thompson while robbing a pawn shop on Dec. 12, 1998. Prosecutors said Lee entered Jimmy’s Pawnshop with a sawed-off shotgun and shot Ellis, the owner of the store, and Thompson, a store employee.

A jury voted 7-5 that Lee should receive a sentence of life imprisonment. However, a judge overrode that recommendation and sentenced Lee to death. Alabama in 2017 ended the practice of judicial override and no longer allows a judge to disregard a jury’s sentencing decision in death penalty cases.

The ruling came several hours after a vigil was held at the Alabama Capitol urging the governor to reduce Lee’s sentence to life imprisonment.

Alabama Attorney General Steve Marshall said he opposed the clemency request.

“The people of Alabama have not forgotten Jimmy and Elaine. I have not forgotten them,” Marshall said. “Anything short of carrying out the sentence imposed by the court falls short of justice for the victims, and that is not what victims of this state deserve.”