Tennessee has carried out its first execution in nearly a decade using a controversial cocktail of drugs including a lethal ingredient described by the Supreme Court as "chemically burning at the stake".
Billy Ray Irick, an inmate convicted of the 1985 rape and murder of seven-year-old Paula Dyer, received a three-drug injection on Thursday night after the US Supreme Court denied a final request to stay his execution.
The lethal injection consisted of midazolam, used as sedative during an execution, a muscle-relaxer called vecuronium bromide, and compounded potassium chloride, the agent that stops the heart.
Thursday’s execution came amid a pending legal challenge against the state’s lethal injection. In recent years, experts and lawyers for death row offenders have raised questions about Midazolam's effectiveness as a sedative in executions.
Witness accounts also raised questions about whether the inmates were sufficiently anaesthetised when the lethal substances were administered, and suggested they may not be not chemically appropriate.
Justice Sonia Sotomayor said: "Although the Midazolam may temporarily render Irick unconscious, the onset of pain and suffocation will rouse him. And it may do so just as the paralysis sets in, too late for him to alert bystanders that his execution has gone horribly (if predictably) wrong.
"In refusing to grant Irick a stay, the Court today turns a blind eye to a proven likelihood that the State of Tennessee is on the verge of inflicting several minutes of torturous pain on an inmate in its custody," Ms Sotomayor wrote. "If the law permits this execution to go forward in spite of the horrific final minutes that Irick may well experience, then we have stopped being a civilised nation and accepted barbarism."
The US Supreme Court has described potassium chloride as "chemically burning at the stake".
Irick's execution started at 7.26pm. At 7.34pm, he was coughing, huffing and taking deep breaths. He then let out a cough or choking sound and his face turned dark purple. He was pronounced dead at 7.48pm.
His victim's mother and family members watched the execution in a separate viewing room but declined to speak at the news conference or release a statement.
Tennessee Attorney General Herbert Slatery said justice was delayed too long for the Knoxville school girl he had raped and killed while babysitting.
"I hope tonight's lawful execution in some way eases the heartache Paula's family has lived with and brings a degree of closure to a chapter of their lives that has been indescribably difficult," Mr Slatery said in the statement.
Hours before his execution, the 59-year-old's lawyer made a final attempt to save Irick's life, arguing that he had suffered from psychosis for the majority of his life and was mentally ill at the time of the offence.
Gene Shiles told members of the media: "I never thought for one moment it would come to this. I never did. I thought somebody would actually look at the facts. I was wrong.”
Tennessee currently has pending legislation that, if passed, would make it illegal to apply the death penalty to a person with serious mental illness.
Robert Durham, executive director of the Death Penalty Information Center, said: "It's unseemly that Irick would be executed and then the case ultimately gets resolved in his favour."
In his final moments, Irick apologised for his actions, saying: “I just wanted to say I'm really sorry and that, that's it.”
His execution comes a week after Pope Francis revealed new Catholic church teaching that deems the death penalty "inadmissible" under all circumstances.
Irick is the 15th inmate executed this year in the United States and the first in Tennessee since December 2009.
Convicted murdered Cecil Johnson received a lethal injection for the 1980 killings of three people during a Nashville corner shop robbery.