Kenneth Eugene Smith was strapped to a gurney and had a mask placed over his face. Through the mask, a lethal dose of nitrogen gas was administered.

Until then, it was an untested method to execute a death row prisoner, Smith the definition of a human guinea pig.

Before the execution Alabama state experts had suggested it would take 40 seconds for Smith to be rendered unconscious.

But five media witnesses said that for between two and four minutes, he "shook and writhed" on the gurney and for several minutes after that was breathing heavily.

Smith's "spiritual adviser", Reverend Jeff Hood, was inside the execution chamber with Smith offering him final comfort. He described the scene even more viscerally.

"What we saw was minutes of someone struggling for his life," he said.

"Heaving back and forth, we saw spit, we saw all sorts of stuff develop from the mask. The mask was tied to the gurney, ripping his head back and forth over and over again."

But the Alabama department of corrections had a different version of events. Ask them how the experiment went and they'll hail it a success.

"Nothing was out of the ordinary for what we were expecting," commissioner John Hamm said.

"He [Smith] struggled against his restraints a little bit but there's some involuntary movement. So that was all expected and the side effects that we've seen or researched on nitrogen hypoxia."

Hypoxia is a medical term for a state of insufficient oxygen in the body.

Nitrogen, an odourless gas, makes up about 78% of the air inhaled by humans.

Under the method of nitrogen hypoxia, the person breathes in only nitrogen, leading within minutes to death from lack of oxygen.

Read more:
Humane death or lethal experiment? How Alabama's controversial nitrogen execution works

Death row opponents had expressed concern that oxygen might seep under what they called a "one size fits all" mask that Smith was wearing, prolonging his suffering.

But if the execution is deemed to have been a success, in spite of varying accounts from witnesses, it's likely other states will follow.

Three states - Alabama, Mississippi and Oklahoma - have authorised nitrogen hypoxia as an execution method.

Alabama is seeking alternative forms of execution because of several botched attempts at lethal injection, including one on Smith in November 2022.

But some states are also looking for new execution methods because drugs used in lethal injections have become difficult to find.

More than 34 years after Smith became a convicted killer, his demise by nitrogen gas is a new frontier for how America executes its death row prisoners.