A death row inmate in Oklahoma died of a heart attack after his execution was halted because the lethal injection of three drugs failed to work properly.
Clayton Lockett, 38, experienced a vein failure 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.
The execution of fellow inmate Charles Warner, due to take place just two hours later, was postponed for 14 days.
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.
"He was conscious and blinking, licking his lips even after the process began. He then began to seize," Associated Press news agency reporter Bailey Elise McBride tweeted from the scene.
"This was botched, and it was difficult to watch," said David Autry, one of Lockett's lawyers. ?
Warner, 46, had been scheduled to be put to death in the same room two hours later in a rare double execution.
Lockett was sentenced to death for the 1999 shooting of a 19-year-old woman. Warner was convicted for the 1997 murder and rape of an 11-month-old girl.
The two men had unsuccessfully challenged an Oklahoma state law that blocks officials from revealing - even in court - 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".
In recent years US states have had increasing problems in trying to obtain drugs used in executions, amid an embargo by European pharmaceutical firms.
Some have turned to untried combinations of drugs or have sought to obtain the drugs custom-made from compounding pharmacies.