Supreme Court Justice Sonia Sotomayor wrote Tuesday that lethal injection “may turn out to be our most cruel experiment yet” in the search for a humane manner in which to carry out the death penalty.

Sotomayor, along with Justice Stephen G. Breyer, dissented from the court’s decision not to hear the case of Thomas Douglas Arthur, Alabama’s oldest inmate, who killed his girlfriend’s husband in 1982. The court, with Chief Justice John G. Roberts Jr. providing a “courtesy” fifth vote, recently delayed Arthur’s execution while deciding whether to take his case.

As is the court’s custom, the majority did not give a reason for passing up Arthur’s case.

Sotomayor’s dissent was just the latest example of how the court is split over the death penalty, although clearly a majority of the court still finds it constitutional. Breyer and Justice Ruth Bader Ginsburg have called for the court to accept a case that would allow briefing on that question but have found no other takers.

Sotomayor in her Tuesday dissent did not question the constitutionality of the death penalty but criticized the methods by which it is carried out. In the past, she has been especially wary of Alabama’s procedures.

Arthur had objected to the use of the drug midazolam in his planned lethal injection, saying it has led to unconstitutional levels of pain and suffering in previous executions. The Supreme Court upheld the drug’s use in 2015’s Glossip v. Gross, and said objecting inmates must prove the drug would cause severe pain and propose another means of execution.

Arthur proposed a firing squad. But the U.S. Court of Appeals for the 11th Circuit turned him down, saying that he had not met the burden of proving midazolam would cause pain so severe it would be unconstitutional and that the state of Alabama did not sanction firing squads.

Sotomayor called the Glossip decision’s test “a macabre challenge” but said Arthur had met it.

“After 34 years of legal challenges, Arthur has accepted that he will die for his crimes,” Sotomayor wrote. “He now challenges only how the state will be permitted to kill him.”

Sotomayor wrote the dissenting opinion for the court’s liberals in Glossip and Tuesday repeated assertions that midazolam might only mask pain and suffering during lethal injections, not relieve it. The court in the past has said needless suffering during executions is unconstitutional.

Death by firing squad may be more instant and cause less suffering, Sotomayor wrote. But in any case, denying Arthur that choice makes the test the court authorized in Glossip irrelevant, she said. “Under this view, even if a prisoner can prove that the state plans to kill him in an intolerably cruel manner, and even if he can prove that there is a feasible alternative, all a state has to do to execute him through an unconstitutional method is to pass a statute declining to authorize any alternative method,” Sotomayor wrote. “This cannot be right.”

While Sotomayor has not joined the call to consider whether the death penalty can ever be constitutionally applied, she has become an outspoken critic of how it is carried out.

Referring to hanging, electrocution, the gas chamber and lethal injection, she said: “Evolving standards have yielded a familiar cycle: States develop a method of execution, which is generally accepted for a time. Science then reveals that — unknown to the previous generation — the states’ chosen method of execution causes unconstitutional levels of suffering.”

Lethal injection, she said, may be the latest in that pattern.

“What cruel irony that the method that appears most humane may turn out to be our most cruel experiment yet,” she wrote.