MONTGOMERY, Alabama (AP) — A judge has refused to stop the nation’s third scheduled execution by nitrogen gas, which will take place in Alabama later this month.

U.S. District Judge R. Austin Huffaker Jr. on Wednesday denied a request for a preliminary injunction to stop Alabama from executing Carey Dale Grayson on Nov. 21 using the same nitrogen gas protocol. The judge said Grayson failed to meet the high legal burden of showing that he is likely to prevail on his claim that the method is unconstitutionally cruel.

“Their evidence and accusations amount to speculation, a speculative parade of highly improbable events and, at best, scientific controversy. “They fall short of demonstrating that the nitrogen hypoxia protocol creates an unacceptable risk of pain, much less added pain,” Huffaker wrote.

John Palombi, an attorney with the Federal Defender Program, which represents Grayson, said they plan to appeal.

The method of execution involves placing a respirator gas mask over the inmate’s face to replace the breathing air with pure nitrogen gas, causing death from lack of oxygen. Critics have argued that the state’s execution protocol does not provide the quick death that the state says it produces.

Kenneth Smith was executed in January at the first execution in the country with nitrogen gas and Alan Miller was executed last month. Media witnessesincluding The Associated Press, He described how the inmates trembled on the stretcher for two minutes or more, the movements followed by what seemed like several minutes of periodic labored breathing with long pauses between them.

Huffaker issued the ruling after a hearing in which Alabama’s corrections commissioner and others testified about what they saw in the first nitrogen gas executions. Grayson’s attorneys presented news articles from media execution witnesses that described the two men’s movements during the execution.

Huffaker said that “the evidence about what actually happened, or what witnesses observed during Smith’s execution, was contradictory and inconsistent.”

“But what that evidence demonstrated was that the nitrogen hypoxia protocol was successful, resulting in death in less than 10 minutes and loss of consciousness in even less time,” Huffaker wrote.

Grayson was one of four teenagers convicted of the 1994 murder of Vickie Deblieux, 37, in Jefferson County. Prosecutors said Deblieux was hitchhiking from Tennessee to her mother’s home in Louisiana when the teens offered her a ride. Prosecutors said they took her to a wooded area, attacked her, threw her off a cliff and then mutilated her body.

Grayson is the only one facing the death penalty. Two other teenagers had their death sentences overturned when the U.S. Supreme Court banned the execution of criminals who were under 18 at the time of the crime. Grayson was 19 years old.

Lethal injection remains the state’s primary method of execution, but inmates can request to be executed with nitrogen gas or the electric chair.