The state of Alabama executed a man through nitrogen gas suffocation Thursday evening after the U.S. Supreme Court declined to intervene, marking the state's seventh execution using the controversial method that an increasing number of states have adopted in recent years.

Anthony Boyd, who was sentenced to death for taking part in the 1993 murder of Gregory Huguley, was pronounced dead at 6:33 p.m. CT after inhaling pure nitrogen gas through a tightly fitted mask inside William C. Holman Correctional Facility in Atmore, Alabama, officials said.

In a statement following the execution, the state Department of Corrections said that Boyd, 54, had nine visitors and made two phone calls on Thursday. He ate breakfast, refused lunch and dinner and declined a final meal request.

On Thursday afternoon, the U.S. Supreme Court turned down Boyd's application to halt the execution. Justice Sonia Sotomayor wrote an opinion dissenting to the denial, joined by Justices Elena Kagan and Ketanji Brown Jackson.

Boyd's execution marks Alabama's seventh nitrogen gas execution since January 2024, when it became the first jurisdiction worldwide to employ the method, also known as nitrogen hypoxia.

In March, Louisiana used nitrogen hypoxia to execute Jessie Hoffman Jr. Arkansas, Mississippi and Oklahoma have also authorized the method, but have not yet used it.

In 1995, a state jury convicted Boyd of capital murder and recommended the death penalty by a 10-2 vote for being among the four people who kidnapped Huguley at gunpoint, taped him to a bench and burned him to death over a $200 cocaine debt. No phys­i­cal evi­dence linked Boyd to the crime, and his prosecution rested on the account of a witness who testified for the state in exchange for a plea deal. Boyd has main­tained his inno­cence throughout his incarceration.

At the time Boyd was sentenced, Alabama's default method of execution was electrocution. In 2002, the state switched to lethal injection. Against a backdrop of yearslong litigation focusing on lethal injection protocols, Alabama lawmakers amended state law in June 2018 to allow nitrogen hypoxia as an authorized alternative method of execution. That same month, Boyd opted into it.

Under Alabama's nitrogen hypoxia protocol, the person facing execution is brought to the chamber, secured to a gurney and fitted with an industrial-use respirator. After officials read a death warrant and the condemned is given a chance to say a few final words, pure nitrogen begins to flow into the mask for 15 minutes or until five minutes have elapsed after an electrocardiogram shows a flatline, whichever period is longer.

Alabama first used the method on Jan. 25, 2024, to execute Kenneth Smith.

On Oct. 20, the U.S. Court of Appeals for the Eleventh Circuit shut down a lawsuit challenging Alabama's nitrogen hypoxia protocol under the Eighth Amendment, which prohibits "cruel and unusual punishments."

To be successful in an execution method challenge under the amendment, Supreme Court precedent set in Baze v. Rees and Glossip v. Gross requires a petitioner to show that there is a readily available alternative method that would significantly reduce the risk of "superadded" pain, and that the state has refused to use the proposed alternative methods without a valid reason.

In the lawsuit originally filed on July 16 in a federal district court in Montgomery, Boyd argued that nitrogen hypoxia creates a risk of conscious suffocation that inflicts "extreme pain and terror." He pointed to people previously executed with nitrogen gas who were seen physically struggling against their restraints as evidence that they were still conscious and experiencing distress after inhaling the gas.

As alternatives, Boyd identified death by firing squad as his first choice, followed by hanging and medical aid in dying, or MAID, which involves lethal doses of medication prescribed by a doctor to people who meet certain criteria, usually terminal illness. He later dropped hanging as one of the options.

At an evidentiary hearing in early September, medical experts summoned by Boyd testified about an "air hunger" akin to an asthma attack that a person may experience once the flow of nitrogen gas begins to deprive the person of oxygen. Other witnesses told the court about what they described as signs of distress — shaking and jerking, convulsing and gasping for air — shown by other people killed using the method.

One of the experts testified that a person who breathes normally might fall unconscious within two minutes while inhaling the gas, but that time could be longer if a person breathes shallowly or holds their breath.

On Oct. 9, U.S. District Judge Emily C. Marks of the Middle District of Alabama held that a person's longer time to unconsciousness might be "an involuntary response to the knowledge that he is about to die," and said it was not evidence that the nitrogen gas protocol is defective.

She added that Boyd did not establish that his alternatives would reduce the risk of suffering, concluding that a firing squad would be more painful and that MAID was not feasible because it would require additional medical staff and personnel.

The Eleventh Circuit agreed with the lower court, leaving Boyd's hopes in the hands of a Supreme Court that has been consistently reluctant to halt executions, except in extraordinary circumstances, or to disturb states' capital punishment policies in general. The justices ultimately allowed Boyd's execution to move forward.

In her dissenting opinion, Justice Sotomayor said the nitrogen gas would add "additional and unnecessary psychological terror" for Boyd.

"Boyd asks for the barest form of mercy: to die by firing squad, which would kill him in seconds, rather than by a torturous suffocation lasting up to four minutes," she wrote. "The Constitution would grant him that grace. My colleagues do not."

--Editing by Alanna Weissman.