In August, Alabama Attorney General Steve Marshall, a Republican, asked the court to allow the state to proceed with gassing Kenneth Smith, who was convicted of murder in 1996, using a face mask connected to a cylinder of nitrogen intended to deprive him of oxygen.

Smith, 58, is one of only two people alive in the U.S. to have survived an execution attempt after Alabama botched his previously scheduled execution by lethal injection in November when multiple attempts to insert an intravenous line into a vein failed.

Smith's lawyers have said the untested gassing protocol may violate the U.S. Constitution's ban on "cruel and unusual punishments," and have argued a second attempt to execute him by any method is unconstitutional. They also told the court that Smith had not yet exhausted his appeals.

In a brief order issued on Wednesday, the court, whose justices are all Republicans, said Alabama Governor Kay Ivey, a Republican, must set a date for the state's Department of Corrections to execute Smith. Two justices dissented from the order and one recused.

A spokesperson for Ivey said her office had not yet determined a date. A spokesperson for the Department of Corrections said the department is "prepared to carry out the orders of the court."

Smith was one of two men convicted in the 1988 murder-for-hire slaying of Elizabeth Sennett in Alabama's Colbert County.

"Elizabeth Sennett's family has waited an unconscionable 35 years to see justice served," Alabama Attorney General Steve Marshall, a Republican, said in a statement. "Though the wait has been far too long, I am grateful that our talented capital litigators have nearly gotten this case to the finish line."

Most U.S. executions are carried out using lethal doses of a barbiturate, but some states have struggled to obtain the drugs because of a European Union law banning pharmaceutical companies from selling drugs that can be used in executions to prisons, forcing them to contemplate other methods.

Oklahoma and Mississippi have also approved nitrogen asphyxiation executions, but have yet to try the method.

The state earlier released a heavily redacted version of the new gassing protocol, which was first authorized by state lawmakers in 2018.

Death penalty experts say it does not make clear how executioners will ensure no oxygen seeps into the mask they intend to put on Smith's face for at least 15 minutes, nor enough information about how the state will mitigate the danger to execution officials and others of using an invisible, odorless gas inside the death chamber.

Robert Grass, a lawyer for Smith, said in a statement that "we remain hopeful that those who review this case will see that a second attempt to execute Mr. Smith - this time with an experimental, never-before-used method and with a protocol that has never been fully disclosed to him or his counsel - is unwarranted and unjust."

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