The U.N. Human Rights Office and the European Union on Friday condemned the execution of Kenneth Eugene Smith with nitrogen gas, a previously untested method of capital punishment that's drawn widespread scorn and outrage.

Smith, 58, was pronounced dead at 8:25 p.m. Thursday in an execution that lasted about 22 minutes. With a mask over his face pumping in pure nitrogen gas, Smith appeared to convulse for several minutes after the gas was turned on.

“He was writhing and clearly suffering,” Ravina Shamdasani, spokesperson for the U.N. Human Rights Office, said at a regular U.N. briefing in Geneva. “Rather than looking for novel, untested methods to execute people, let’s just bring an end to the death penalty. This is an anachronism that doesn’t belong in the 21st century.”

The U.N. Human Rights Office had previously warned officials that it believed the method, known as nitrogen hypoxia, "could breach the prohibition on torture or other cruel, inhuman or degrading treatment or punishment."

In a statement on Friday, the European Union said nitrogen hypoxia was "particularly cruel and unusual punishment" and called for states to "move toward abolition, in line with the worldwide trend."

Also on Friday, Alabama Attorney General Steve Marshall said the execution was a "success" and described it as "textbook." He told reporters that nothing unexpected occurred during the execution, including Smith's "involuntary movements."

“As of last night, nitrogen hypoxia as a means of execution is no longer an untested method – it is a proven one,” he said. “To my colleagues across the country … Alabama has done it and now so can you. And we stand ready to assist you in implementing this method in your states.”

He said Alabama "will definitely have more nitrogen hypoxia executions," adding that 43 death row inmates in the state have already elected the newly tested method.

Nitrogen hypoxia is the latest method of capital punishment implemented in the U.S. since lethal injection was introduced in 1982. Alabama officials called the method humane but others, including three Supreme Court justices, said more should've been known about the method before it was used. In her dissent of the Supreme Court's rejection of Smith's recent appeal on Wednesday, Justice Sonia Sotomayor mentioned Alabama's failed attempt to execute Smith by lethal injection in 2022.

“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 said. “The world is watching.”

Smith was one of two men convicted in the 1988 murder-for-hire slaying of Elizabeth Sennett in northwestern Alabama. Prosecutors said the men were paid $1,000 to kill Sennett on behalf of her pastor husband Charles Sennett, who wanted to collect on insurance to pay debts. Charles Sennett died by suicide after learning he was a suspect in the crime.

The other man, John Forrest Parker, 42, was executed by lethal injection in June 2010. Smith's initial conviction was overturned but in 1996 he was convicted again and sentenced to death.

Amid a shortage of drugs used in lethal injections, states have been searching for new execution methods. Alabama, Oklahoma and Mississippi have authorized the use of nitrogen hypoxia for capital punishment, but Alabama was the first to carry out an execution using the method.

Contributing: Associated Press; Jeanine Santucci, Thao Nguyen, Maureen Groppe