Alabama carried out the first-ever execution by nitrogen gas Thursday, killing 58-year-old Kenneth Smith just over a year after he survived a failed lethal injection.

Smith “appeared to shake and writhe on the gurney, sometimes pulling on the restraints” for at least two minutes, followed by several minutes of heavy breathing, according to The Associated Press. He was pronounced dead at 8:25 p.m. local time.

Rev. Jeff Hood, Smith’s spiritual adviser, described the execution as “torture.”

“It was the most horrible thing I think I’ve ever seen,” said Hood, who has witnessed five execution in the past 13 months. “I stood there and cried while I saw someone get suffocated.”

“Tonight Alabama causes humanity to take a step backwards,” Smith said in his final statement, according to media witnesses. “Thank you for supporting me. Love you all.”

In Smith’s final days, his lawyers had asked the Supreme Court to block the execution. They argued that the second attempt to kill Smith — with an untested mechanism, before he had exhausted state court appeals, and while he was still experiencing symptoms of post-traumatic stress disorder from the first attempt — would violate Eighth Amendment protections against cruel and unusual punishment.

The court denied the request, with the three liberal justices, Sonia Sotomayor, Elena Kagan and Ketanji Brown Jackson dissenting. The conservative majority did not provide a written explanation for the decision.

“Having failed to kill Smith on its first attempt, Alabama has selected him as its ‘guinea pig’ to test a method of execution never attempted before,” Sotomayor wrote in her dissent. “The world is watching.”

“Twice now this Court has ignored Smith’s warning that Alabama will subject him to an unconstitutional risk of pain. The first time, Smith’s predictions came true,” Sotomayor said. “This time, he predicts that Alabama’s protocol will cause him to suffocate and choke to death on his own vomit. I sincerely hope that he is not proven correct a second time.”

As Smith’s execution approached, he vomited continuously for several days in a row, his lawyers said in court filings, citing his post-traumatic stress disorder from his first execution date. In response, the Alabama Department of Corrections said it would deny Smith solid foods, starting at 10 a.m. Thursday, to minimize the risk that he would vomit into the gas mask.

Smith is the second person in U.S. history to face a second execution attempt after a failed effort, his lawyers said. The first was in 1946 in Louisiana, when an electric chair malfunctioned. Smith’s death marked the first known nitrogen gas execution, properly referred to as nitrogen hypoxia, in the world.

The Alabama attorney general’s office claimed without evidence in a court filing response that nitrogen is “perhaps the most humane method of execution ever devised.” The limited information that exists about nitrogen as a killing agent comes from euthanizing small animals as well as studying industrial accidents and suicides. The American Veterinary Medical Association said in its 2020 euthanasia guidelines that nitrogen hypoxia should not be used to kill most mammals and that rats who were exposed to nitrogen gas showed signs of “panic and distress” before they collapsed and died.

Alabama did not kill Smith with nitrogen because of a desire to make his death more pleasant. The state first authorized nitrogen executions in 2018 amid drug shortages and legal challenges to its existing lethal injection procedure. The decision to actually conduct an execution with nitrogen came after the state failed to execute Smith by lethal injection in November 2022 — the third consecutive botched lethal injection in Alabama that year.

“The eyes of the world are on this impending moral apocalypse,” Smith and Hood, his spiritual adviser, said in a joint statement Thursday morning. “Our prayer is that people will not turn their heads. We simply cannot normalize the suffocation of each other.”

Hood, who was in the execution chamber with Smith to deliver last rites, was required to sign a waiver acknowledging the risk that he could be exposed to the odorless, tasteless nitrogen gas.

Smith was killed as punishment for participating in a 1988 murder-for-hire plot to kill a pastor’s wife. The pastor, Charles Sennett Sr., wanted his wife, Elizabeth Sennett, killed in an apparent robbery so he could collect an insurance payout. A jury recommended 11-1 that Smith, who confessed to helping plan the hit but denied stabbing the victim, be sentenced to life in prison without the possibility of parole. The judge overrode the jury and imposed a death sentence.

Sennett’s son Mike Sennett said at a news conference after the execution that his mother “got her justice tonight” and that “evil deeds” had been paid.

Had Smith’s trial taken place today, he would not have been sentenced to death. In 2017, Alabama became the last state to ban judges from overriding jury recommendations to impose death sentences.

During Alabama’s first attempt to kill Smith on Nov. 17, 2022, he lay strapped to the gurney, unable to move for nearly four hours. While on the gurney, he was unaware that a federal appellate court had issued a stay of execution, which the Supreme Court soon vacated. Once his appeals were exhausted, execution officials unsuccessfully tried to set two IV lines, sliding a needle in and out of his arms and hands and ignoring his complaints of pain, his lawyers wrote in a complaint filed in federal court shortly after the execution attempt.

Then, the IV team tilted the gurney back, forcing Smith into a “reverse crucifixion position with his head below his feet,” the complaint said. He was jabbed five or six times with a clear syringe and poked multiple times with a large-gauge needle in his collarbone. Smith was in so much pain that he resisted the restraints, injuring his shoulder and struggling to breathe.

“They were just sticking me over and over, going in the same hole like a freaking sewing machine,” Smith told NPR in an interview last year. “I was absolutely alone in a room full of people, and not one of them tried to help me at all — and I was crying out for help.”

When the execution was finally called off, Smith was unable to stand, walk or dress himself unassisted, the complaint said. He experienced severe post-traumatic stress disorder symptoms, including nightmares, hypervigilance, hyperarousal and dissociation up until his death.

Before his first execution date, Smith sued Alabama Department of Corrections Commissioner John Hamm, alleging that the state’s lethal injection procedure violated the Eighth Amendment right to be free from cruel and unusual punishment. The Supreme Court has previously held that for a challenge to an execution method to prevail, the prisoner must select an alternative method that is feasible, available and reduces the risk of pain.

Smith identified nitrogen hypoxia as an alternative, but the state argued it was not an “available” alternative.

“Then, on the eve of being required to disclose information regarding its failed attempt to execute Mr. Smith by lethal injection, ADOC suddenly changed course, now claiming it is prepared to carry out executions using nitrogen hypoxia,” Smith’s lawyers wrote in the complaint.

The lawyers warned that the one-size-fits-all mask used to deliver the nitrogen gas may not form an adequate seal and could allow oxygen into the mask. If that occurred, they said, Smith could experience a prolonged, painful death, stroke or a persistent vegetative state.

The lawyers also cited concerns with the purity of the nitrogen and a lack of clarity around how it would be stored to prevent contamination. Airgas, a gas distributor in Alabama, previously said it would not supply nitrogen for executions, according to AL.com, a local news outlet.

Although Smith proposed nitrogen as an alternative to lethal injection, he did not agree to be killed under a process that “was hastily introduced as a means to moot Mr. Smith’s pending litigation about [the Alabama Department of Corrections’] previous failed attempt to execute him by lethal injection and forestall discovery into it,” his lawyers wrote.

Alabama Gov. Kay Ivey (R) said in a statement that “after more than 30 years,” Smith “got what he asked for, and this case can finally be put to rest.”

United Nations experts warned earlier this month that nitrogen executions likely violate international prohibitions on torture.

“We are all complicit as Alabama moves forward with a state-sponsored killing that evokes troubling memories of the Holocaust,” said Miriam Krinsky, the executive director Fair and Just Prosecution, a group of progressive prosecutors.

“As a civilized nation, our desire for retribution should never outweigh our humanity,” she said in a statement. “Today we fail that basic test. We unequivocally condemn this execution and once again urge policymakers across the country to abolish the barbaric use of capital punishment.”

Taiyler S. Mitchell contributed reporting.