Arkansas executed its fourth inmate in eight days on Thursday, administering a lethal injection to a man who killed two people after escaping from a prison where he had been serving a life sentence for murdering a cheerleader.
Kenneth Williams, 38, was pronounced dead at 11:05 pm at the state's Cummins Unit prison. The state, which had not held an execution in 12 years until this month, had already put three other inmates to death since April 20.
Pool media reports show that after the lethal injection process began, Williams was shaking and appeared to be gasping for air.
Arkansas does not have a schedule to execute any more inmates.
Arkansas had initially planned to execute eight inmates in 11 days in April, the most of any state in so short a period since the death penalty was reinstated in 1976. Four of those executions were halted by various courts.
The unprecedented schedule was set by Republican Governor Asa Hutchinson because a drug in the state's execution mix, the sedative midazolam, expires at the end of April. The quick pace prompted criticism that Arkansas was acting recklessly.
Midazolam has been used in troubled executions in Oklahoma and Arizona, where witnesses said inmates appeared to twist in pain on death row gurneys.
Critics say the valium-like drug is not suitable to render a person unconscious for surgery and should not be used in executions, which use drugs to stop breathing and the heart.
The Arkansas execution flurry also set off legal filings that raised questions about US death chamber protocols, troubled prosecutions and difficulties in obtaining lethal injection drugs.
The US. Supree Court rejected motions to halt the executions on grounds such as the failure of Arkansas "to provide Mr Williams a forum to litigate his claim that he is intellectually disabled."
Williams, sentenced to life without parole for the 1998 murder of 19-year-old college cheerleader Dominique Hurd, broke out of a maximum-security prison in 1999 after serving less than a month behind bars.
He murdered Cecil Boren, 57 at his farmhouse, shooting him numerous times, before stealing Boren's pickup truck to flee to Missouri, where he slammed his vehicle into one driven by delivery man Michael Greenwood, 24, killing him.
He was sentenced to death for Boren's murder. Boren's family thanked the governor, the state's attorney general and prison officials for "flawless care of this matter," a spokesman for the attorney general said.
But Greenwood's daughter, Kayla Greenwood, urged Hutchinson to spare Williams, in a letter sent on Thursday.
"His execution will not bring my father back or return to us what has been taken, but it will cause additional suffering," she said.