Inmate Clayton Lockett Left Writhing In Agony After Execution Goes Horribly Wrong In Oklahoma Prison

An inmate was left writhing in agony after an experimental drug execution went horribly wrong.

The execution of Clayton Lockett was dramatically halted on Tuesday night in Oklahoma after he started moving and mumbling 13 minutes after being given a supposedly lethal injection.

The killer, who was sentenced to death for shooting a 19-year-old woman and watching his friends bury her alive, was pronounced dead around 25 minutes later after apparently suffering a massive heart attack.

Inmate Clayton Lockett

Witness Ziva Branstetter described how Lockett was thrashing about and appeared to be in pain.

"His body was sort of bucking," she told broadcaster MSNBC.

"He was clenching his jaw. Several times he mumbled phrases that were largely unintelligible."

Oklahoma Department of Corrections director Robert Patton said the inmate was writhing on the gurney and shaking uncontrollably minutes into the execution.

The inmate then sat up and said "something's wrong" before his vein reportedly "exploded," where the drug was administered.

Another inmate, Charles Warner, who was due to be given a lethal injection shortly after Lockett, has been given a 14-day stay of execution while a full review is carried out.

The 46-year-old was convicted of raping and killing his roommate's 11-month-old daughter in 1997.

Charles Warner was also due to be executed

Warner's attorney, Madeline Cohen, said that further legal action can be expected given how "something went horribly awry" Tuesday.

"Oklahoma cannot carry out further executions until there's transparency in this process," Cohen said. "...I think they should all be looking at themselves hard. Oklahoma needs to take a step back."