An Alabama death row inmate on Thursday evening became the first person to be executed in the U.S. using nitrogen gas.

The big picture: Kenneth Eugene Smith made several appeals, including to the U.S. Supreme Court, attempting to halt the state's plan to execute him using the previously untested method.

• Human rights experts had warned it could be "painful and humiliating," but Smith's appeals were rejected.
• The Supreme Court on Wednesday rejected Smith's request for a delay in carrying out the execution, and denied his petition for the high court to review the case. On Thursday, it rejected his request for a stay, with dissents from the court's three liberal justices.

Of note: It's rare for the high court to halt an execution.

• Justice Sonia Sotomayor wrote in the dissent Alabama has selected Smith "as its 'guinea pig' to test a method of execution never attempted before." She added: "The world is watching."

Details: Smith was pronounced dead at 8:25pm local time, officials said.

• On Thursday, journalists present at his execution reported Smith saying in a final statement: "Tonight Alabama causes humanity to take a step backwards. ... I'm leaving with love, peace and light."
• Alabama Corrections Commissioner John Hamm at a Thursday night news conference addressed reports that Smith was "shaking and writhing" for some two minutes at the start of the execution.
• "It appeared that Smith was holding his breath as long as he could," Hamm said. "He struggled against the restraints a little bit but it's an involuntary movement and some agonal breathing," he said.
• "That was all expected and was in the side effects that we've seen or researched on nitrogen hypoxia," Hamm added. "Nothing was out of the ordinary from what we were expecting."

Context: Smith was convicted of capital murder for the killing of Elizabeth Sennett, 45, and sentenced to death in 1996.

• A previous execution attempt via lethal injection failed after Alabama Department of Corrections officials couldn't access his veins.

Zoom in: Smith said in a court document that a second attempt to execute him following the failed one "would violate his right to be free from cruel and unusual punishment."

• The method — administering nitrogen gas via a mask and breathing tube, slowly depriving oxygen — had previously never been used in an execution in the U.S. Human rights experts have said it could ultimately be degrading or torturous.
• Nitrogen gas has been used in assisted suicides in Europe and Australia, but not via a mask, per the New York Times. A leak could be more likely this way than with the tested methods of a room, pod or plastic bag.
• "I am worried that we have told Alabama that these risks could happen — will happen — just like we warned them last year," Smith told the New York Times in an email. "And they will do nothing to prevent these dangers from happening."

What they're saying: "The execution was lawfully carried out by nitrogen hypoxia, the method previously requested by Mr. Smith as an alternative to lethal injection," Alabama Gov. Kay Ivey said in a statement Thursday.

• "At long last, Mr. Smith got what he asked for, and this case can finally be put to rest."

Go deeper: Alabama inmate will be first person in U.S. executed via nitrogen gas

Editor's note: This article has been updated with details throughout.