The governor of the American state of Oklahoma, Mary Fallin, has ordered a full investigation into a prison execution that went terribly wrong.
A death row inmate in the United States died of a heart attack after a botched execution which forced a delay in the execution of his fellow inmate.
Clayton Lockett, 38, experienced a vein failure on Wednesday which prevented the drug cocktail from being fully effective.
The execution was halted after 20 minutes, during which he writhed and shook uncontrollably, US media report.
A spokesman for the Oklahoma Department of Corrections told US media that Lockett had died of a heart attack following injection of three lethal drugs.
"We believe that a vein was blown and the drugs weren't working as they were designed to. The director ordered a halt to the execution," corrections department spokesman Jerry Massie said.
Prison officials pulled a curtain across the view of witnesses when it became apparent that something had gone wrong.
"This was botched, and it was difficult to watch," said David Autry, one of Lockett's lawyers.
Lockett was sentenced to death for the 1999 shooting of a 19-year-old woman.
The execution of fellow inmate Charles Warner, which had been due to take place just two hours later, was postponed for 14 days.
The two men had unsuccessfully challenged an Oklahoma state law that blocks officials from revealing the identities of the companies supplying the drugs used to sedate inmates, paralyse their respiratory systems and stop their hearts.
The state maintains the law is necessary to protect the suppliers from legal action and harassment.
Lockett and Warner argued it was necessary for the men to learn the name of the suppliers in order to ensure the quality of the drugs that would be used to kill them and to be certain that they had been obtained legally.
In March a trial court ruled in their favour, but the state's highest court reversed that decision last week, ruling that "the plaintiffs have no more right to the information they requested than if they were being executed in the electric chair".