A Texas death row inmate complained of a burning sensation as he was executed by lethal injection, exclaiming: "Ooh-ee! I can feel that."
Anthony Allen Shore, 55, became the first convict to be executed in the US this year on Thursday evening.
Known as the Tourniquet Killer, he apologised to his victims before the fatal drug jab of pentobarbital.
The US constitution bans cruel and unusual punishment. Critics say lethal injections violate that legal standard.
Judges in several US states have issued temporary prohibitions on this execution method, after inmates appeared to suffer during the injections.
Shore admitted to killing a woman and three girls, and requested the death penalty at trial.
He was given his grim moniker because of his method of strangling his victims in Houston with a homemade tourniquet.
All four of his victims were Hispanic, murdered over a nine-year period starting in 1986.
Three of them were raped.
They were Laurie Lee Tremblay, 14; Maria Del Carmen Estrada, 21; Diana Rebollar, nine; and Dana Sanchez, 16.
Shore was convicted of murder in 2004 after his DNA, which police had taken when he was convicted of child abuse in 1998, was found on one of his victims.
While strapped to the execution gurney, his voice cracked with emotion as he apologised to his victims, according to an Associated Press (AP) reporter who witnessed his death.
"No amount of words or apology could ever undo what I've done," Shore said.
"I wish I could undo the past, but it is what it is."
Thirteen minutes after the injection, he was declared dead.
AP reporter Mike Graczyk told reporters afterwards: "He said you could feel it, it was hot, it was burning."
"He goes, 'Ohhh, ooooh wee,' is how he put it," he added.