A day after Alabama became the first state to execute a prisoner with nitrogen gas, officials vowed on Friday to continue using the method in executions despite witnesses’ accounts that the prisoner writhed on the gurney for at least two minutes.

Two very different accounts of the execution emerged from the state’s death chamber in Atmore, Ala., where the state executed Kenneth Smith, 58, on Thursday night.

The state’s attorney general, Steve Marshall, called it a “textbook” execution that had made nitrogen hypoxia, as the process is known, a “proven” method that other states could emulate.

“Alabama has done it, and now so can you,” Mr. Marshall said, addressing his counterparts across the country. “And we stand ready to assist you in implementing this method in your states.”

Meanwhile, Mr. Smith’s spiritual adviser and reporters who also witnessed the execution described an intense reaction in which Mr. Smith violently shook and writhed as the gas was administered, began breathing heavily and, finally, stopped moving.

The descriptions were at odds with what the state had promised in court papers: that the untested method of using nitrogen gas through a face mask would “rapidly lower the oxygen level in the mask, ensuring unconsciousness in seconds.”

“It was appalling,” said Deborah Denno, an expert on execution methods at Fordham University Law School. “Pain for two to four minutes, particularly when you’re talking about somebody who’s suffocating to death — that’s a really long period of time and a torturous period of time.”

Mr. Marshall gave the go-ahead to prison officials to begin pumping nitrogen into Mr. Smith’s mask at 7:56 p.m., about a minute before witnesses reported that the prisoner began writhing uncontrollably.

Lee Hedgepeth, a reporter in Alabama who witnessed the execution, wrote a detailed account of his observations in which he said that Mr. Smith began “thrashing against the straps” on the gurney at 7:57 p.m., “his whole body and head violently jerking back and forth for several minutes.”

Next, Mr. Hedgepeth wrote, Mr. Smith began heaving, and by 8 p.m., he was still gasping for air, his body pulling at the restraints with each gasp, though less forcefully.

Criticism of the execution poured in from around the world from such organizations as the United Nations High Commissioner for Human Rights, the European Union and Amnesty International. The White House press secretary said the Biden administration was “deeply troubled” by the accounts of Mr. Smith’s death.

Mr. Marshall said that 43 other prisoners currently on death row in the state have opted for the nitrogen hypoxia method under a law that was passed several years ago, allowing them to choose that method over lethal injection. The state has notoriously botched a string of lethal injections, including one attempt to execute Mr. Smith in 2022.

He was one of three men convicted in the 1988 murder of a woman, Elizabeth Sennett, whose husband, a pastor, had recruited them to kill her.

“I think we will definitely have more nitrogen hypoxia executions in Alabama,” Mr. Marshall said.

Over the past 15 years or so, states have had to contend with an embarrassing string of bungled executions and the increased difficulty of obtaining the drugs needed for lethal injections. Some have contemplated the pros and cons of older methods like electrocution and firing squads, while others have seen more promise in new drug cocktails or nitrogen hypoxia, which suffocates the prisoner by replacing air with pure nitrogen.

Still other states, including New Hampshire, Colorado and Virginia, have abolished the death penalty entirely. (In all, 27 states and the federal government have the death penalty.) While a majority of Americans still approve of capital punishment, the level of support for it has dwindled from 80 percent in 1994 to the mid-50s in recent years, according to Gallup. Last November, Gallup found that half of Americans believe the death penalty is applied unfairly, a record high.

Experts said that support for the death penalty declines when executions are botched or methods are viewed as unusual or inhumane.

Alabama is one of three states — Oklahoma and Mississippi are the others — that have authorized the use of nitrogen in executions. While the gas has been used in physician-assisted suicides, the method chosen by Alabama — administering the gas through a mask — differs from common practice and had raised concerns that a leak could endanger others present in the death chamber; that Mr. Smith could vomit into his mask; or that oxygen could mix with the nitrogen.

Robin Maher, the executive director of the Death Penalty Information Center, said that Mr. Smith’s death had by no means demonstrated the reliability of the method, and that human error is always a factor. “The risk is baked into this procedure, and there’s going to be no way to know if it’s going to go like this next time, if it will be worse or if it will be better,” she said.

She added that she did not think other states would jump to follow suit. “I would hope and expect that other states are not going to want to take on the risks that Alabama took on,” she said.

One state that is considering doing so is Nebraska, where the Legislature abolished the death penalty in 2015 but voters restored it in a referendum the following year. Soon after, the state saw its stockpile of lethal injection drugs expire, and it has been unable to carry out executions.

“Given the outcome of the Alabama case, we are confident that this will be a highly debated bill in our state,” said State Senator Loren Lippincott, a Republican and sponsor of a bill that would approve the use of nitrogen, in a written statement. “If given this option, we are certain that the Nebraska Department of Corrections will use this method to humanely give justice to victims’ families and our community.”

The Rev. Jeff Hood, an Arkansas-based pastor who was the spiritual adviser in the execution chamber with Mr. Smith, challenged the idea that the execution had gone as officials anticipated.

He said he saw prison officials in the chamber who appeared “visibly surprised at how bad this thing went.”

Experts said it was almost the rule that execution methods heralded as humane or painless turn out to be far more complicated — either intrinsically, as with the use of cyanide gas, or through human error. Autopsies and execution logs have repeatedly suggested that executed prisoners were not given enough sedative to render them unconscious.

There will always be a debate over the experience of executions and how much prisoners like Mr. Smith suffered, Ms. Maher of the Death Penalty Information Center pointed out.

“The one person who can tell us about that is now dead,” she said.