(CNN)- Alabama inmate Kenneth Smith was put to death Thursday night by nitrogen hypoxia, marking the nation’s first known execution using that method. Now the state’s attorney general wants to help other states interested in using the new form of capital punishment.

Smith, 58, was sentenced to death for his role in a 1988 murder for hire and had previously survived a failed attempt to execute him by lethal injection in 2022. His attorneys fought the execution until the end, ultimately losing a final appeal to the US Supreme Court on Thursday evening.

Smith’s legal team – alongside experts and advocates from the US to the United Nations – had voiced concern that nitrogen hypoxia could lead to excessive pain or even torture.

Alabama is one of only three states – Oklahoma and Mississippi being the others – to have approved the method, which is designed to replace oxygen in the body with a high concentration of nitrogen, causing death. So far, Alabama is the only state to have carried it out or even outlined a protocol on how to do it.

Here’s what we know about the historic execution:

In Alabama, 43 other inmates have requested new execution method

After widespread discussion and speculation about how Smith’s execution would unfold, Alabama Attorney General Steve Marshall told reporters on Friday, “What occurred last night was textbook.”

“As of last night, nitrogen hypoxia as a means of execution is no longer an untested method. It is a proven one. It’s the method that Kenneth Smith ultimately chose, along with 43 other death row inmates in our state,” Marshall said.

“I now suspect many states will follow.”

The attorney general also had a message for colleagues across the country who were monitoring developments in the nation’s first execution using nitrogen hypoxia:

“Alabama has done it, and now so can you,” Marshall said. “And we stand ready to assist you in implementing this method in your states.”

What happened during the execution

As the procedure started Thursday evening at the William C. Holman Correctional Facility in Atmore, Smith was fitted with a mask, a device that would be used to administer the nitrogen.

The execution process began at 7:53 p.m. CT Thursday, and Smith was pronounced dead at 8:25 p.m., according to Alabama Department of Corrections officials.

Nitrogen flowed for about 15 minutes during the procedure, state corrections commissioner John Hamm said in a news conference.

Smith, who was on a gurney, appeared conscious for “several minutes into the execution,” and “shook and writhed” for about two minutes after that, media witnesses said in a joint report.

That was followed by several minutes of deep breathing before his breath began slowing “until it was no longer perceptible for media witnesses,” the media witnesses said.

When asked at the news conference about Smith shaking at the beginning of the execution, Hamm said Smith appeared to be holding his breath “for as long as he could” and may have also “struggled against his restraints.”

“There was some involuntary movement and some agonal breathing, so that was all expected and is in the side effects that we’ve seen and researched on nitrogen hypoxia,” Hamm said. “So nothing was out of the ordinary of what we were expecting.” Agonal breathing is an irregular, gasping breath pattern that can happen when someone is near death.

Smith’s spiritual adviser, the Rev. Jeff Hood, who’d previously expressed concern that the method could be inhumane, witnessed the execution and described it in more graphic terms, saying it was “the most horrible thing I’ve ever seen.”

Smith, wearing a tight-fitting mask that covered his entire face, convulsed when the gas was turned on, “popped up on the gurney” repeatedly, and gasped, heaved and spat, Hood said.

“It was absolutely horrific,” he said.

Smith made a lengthy statement in front of the witnesses before the execution started, according to the pool reporters.

“Tonight Alabama caused humanity to take a step backward,” Smith said in part, according to the reporters. “I’m leaving with love, peace and light. Thank you for supporting me. Love all of you.”

Smith also “made a ‘I love you’ sign in sign language with one of his hands that was facing the room where his family was witnessing,” the journalists’ joint report said.

It’s unclear exactly how long it took for Smith to die.

During lethal injections, the nation’s most common execution method, the time it takes for an inmate to die varies widely depending on the drugs used, number of injections and individual inmates’ reactions.

In a typical three-injection protocol, a person can die as quickly as 30 to 60 seconds after the final fatal injection, experts say. But the process does not always go as outlined. In 2014, for example, an Oklahoma inmate had an apparent heart attack 43 minutes after receiving the first injection, according to state documents and witnesses.

A controversial form of capital punishment

A tense debate has unfolded about whether America’s new execution method is humane and whether the procedure would cause undue pain.

In theory, the method involves replacing the air breathed by an inmate with 100% nitrogen, depriving the body of oxygen.

Proponents argue the process should be painless, citing nitrogen’s role in deadly industrial accidents or suicides, and the state indicated in court records that it believes nitrogen hypoxia is “perhaps the most humane method of execution ever devised.”

Others have been skeptical, fearing the method could go awry and voicing concerns that the state’s plan up until Thursday had been shrouded in secrecy. The protocol published by the state contained several redactions experts said shielded key details from public scrutiny.

Criticism of Alabama’s new death penalty method has spread globally. The United Nations High Commissioner for Human Rights, Volker Turk, said the death penalty is “inconsistent with the fundamental right to life” and called for its “universal abolition.”

“I deeply regret the execution of Kenneth Eugene Smith in Alabama despite serious concerns this novel and untested method of suffocation by nitrogen gas may amount to torture, or cruel, inhuman or degrading treatment,” Turk said in a statement Friday.

The European Union described the new method of execution as “a particularly cruel and unusual punishment” and also called for universal abolition of the death penalty, spokesperson Peter Stano said in a statement Friday.

In recent years, many states that still allow the death penalty have had difficulty obtaining necessary ingredients for lethal injections after European manufacturers refused to sell drugs to the US that would be used in executions.

Stano said the EU welcomed “the fact that 29 US states have either abolished capital punishment or imposed a moratorium on executions.”

Victim’s family is ‘glad this day is over’

he family of Smith’s victim, Elizabeth Sennett, said the execution is “bittersweet” – a final act of justice for the woman killed more than three decades ago.

Sennett was murdered in 1988 after her husband hired someone who hired two others, including Smith, to kill his wife and make it look like a burglary, court records show. The husband, minister Charles Sennett, was having an affair and had taken out an insurance policy on his wife, the records say.

“Nothing that happened here today is going to bring mom back,” said Sennett’s son, Michael, after the execution. But, he added, “we’re glad this day is over.”

Elizabeth Sennett’s sons told CNN earlier Thursday they felt it was time for Smith’s sentence to be carried out, adding they believed their mother had been overlooked due to the focus on the execution method.

“What’s going on is overshadowing what’s actually happened,” Chuck Sennett, said. “He’s gotta pay the price for what he done to our mother,” who should be remembered “as a loving, caring woman.” The brothers were in their 20s when their mother was killed.

The family has forgiven everyone involved in the killing, including Smith, Michael Sennett said at a news conference Thursday night.

“His debt was paid tonight,” the son said.

CNN’s Isabel Rosales, Devan Cole, Christina Maxouris, Lauren Mascarenhas, Alta Spells and Jamiel Lynch contributed to this report.