A judge in Alabama blocked the state from carrying out a scheduled lethal injection execution after the inmate said the state lost his paperwork requesting an alternative method of execution.

U.S. District Judge R. Austin Huffaker, Jr. issued the injunction Monday, saying the state could only move forward with the execution of Alan Miller if it used nitrogen hypoxia. Miller was scheduled to die by lethal injection on Sept. 22.

Miller told the court that he had requested the nitrogen hypoxia method using a state form that was distributed to inmates in 2018. Alabama has yet to finalize state regulations for carrying out executions with the nitrogen method.

Alabama is one of three states in the country that have approved nitrogen hypoxia executions, but none have yet taken place.

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Prisoner Alan Miller

Alan Miller is on death row in Alabama. (Alabama Department of Corrections) (Alabama Department of Corrections)

Alan Eugene Miller getting into police car

Officials escort murder suspect Alan Eugene Miller away from the Pelham City Jail in Ala., on Aug. 5, 1999. Miller, scheduled to be put to death by lethal injection on Sept. 22, 2022, for a workplace shooting rampage in 1999 that killed three men, says the state lost the paperwork he turned in selecting an alternate execution method. (AP Photo/Dave Martin, File) (AP Photo/Dave Martin, File)

Miller, who worked as a delivery driver, murdered three men in a shooting rampage in 1999. Court records say he shot Lee Holdbrooks and Scott Yancy during delivery to one business, then drove off and shot Terry Jarvis at a second business.

A defense psychiatrist said that while Miller displays clear mental instability, it did not rise to the level required for an insanity plea, according to the Associated Press.

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Oklahoma execution chamber

Photo shows the gurney in the execution chamber at the Oklahoma State Penitentiary in McAlester, Okla. (AP Photo/Sue Ogrocki, File)

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Miller's defense team also argued there have been issues with recent executions by lethal injection, such as that of Joe Nathan James Jr., who was put to death in July. The state has admitted that administrators struggled to find a vein to carry out the injection.

The Associated Press contributed to this report.