WASHINGTON —  A convicted murderer vomited and experienced convulsions as he was executed by lethal injection in the United States (US) state of Oklahoma on Thursday (Oct 29), witnesses said.

John Grant, 60, was the first inmate to be put to death in Oklahoma since a series of botched executions led to a temporary moratorium on capital punishment in the state.

Grant, who is Black, was sentenced to death for the 1998 murder of a white prison cafeteria worker, Gay Carter.

Journalists who witnessed the execution said at a press conference that Grant had vomited and experienced full body convulsions about two dozen times before he was pronounced dead.

A federal appeals court had put a hold on Grant's execution over concerns about the drug cocktail used to put inmates to death in the Midwestern state, but the conservative-leaning Supreme Court lifted the last-minute stay and allowed the execution to go ahead.

Attorneys for Grant argued that use of the sedative midazolam would constitute cruel and unusual punishment, violating his constitutional rights.

Midazolam was identified as a potential factor in a series of botched executions in Oklahoma, the last of which was carried out in 2015.

The Oklahoma attorney general's office, however, asked the Supreme Court to vacate the stay and the nation's highest court did so hours ahead of Grant's scheduled execution, with just the three liberal justices objecting.

A lawsuit challenging Oklahoma's lethal injection protocols is scheduled to go to trial in February 2022, and the 10th Circuit Court of Appeals had put executions on hold in the state pending a ruling in the case.

"There should be no more executions in Oklahoma until we go to trial in February to address the state's problematic lethal injection protocol," said Dale Baich, an attorney for Grant and other death row inmates.

For Robert Dunham, executive director of the Death Penalty Information Center, Grant "became a human experiment for the other death-row prisoners' challenge to Oklahoma's execution process."

"Executions like this provide death-penalty opponents with further evidence that states who are in a rush to kill simply cannot be trusted with the death penalty," he told AFP, calling the Supreme Court's decision to vacate the stay "appalling."

'FULL RESPONSIBILITY'

Clayton Lockett, a convicted murderer executed in Oklahoma in April 2014, took more than 40 minutes to die after a drug was injected into muscle tissue instead of his bloodstream.

The wrong drug was used in the execution of Charles Warner the following year and another execution was called off at the last minute when it was discovered the wrong drug was about to be used again.

Ms Sarah Jernigan, Grant's attorney, said her client had taken "full responsibility for the murder of Gay Carter, and he spent his years on death row trying to understand and atone for his actions."

"John never received the mental health care he needed or deserved in prison," Ms Jernigan said.

"And when he eventually committed a violent crime, the murder of a prison worker, Oklahoma provided him with incompetent lawyers who had no business handling a case with the ultimate punishment at stake," she said.

Another Oklahoma death row inmate, Julius Jones, a 41-year-old African American man, is scheduled to be executed on November 18 for the 1999 shooting of a white businessman.

Jones has consistently proclaimed his innocence and his case has attracted the attention of celebrities such as Kim Kardashian and Cleveland Browns quarterback Baker Mayfield. AFP