Photo: The Canadian Press

Arkansas wrapped up an accelerated executions schedule with a lethal injection that left the condemned inmate lurching and convulsing before he died, prompting calls for investigations and renewed scrutiny of the state's efforts to put multiple inmates to death on a compressed timeline.

Kenneth Williams on Thursday became the fourth convicted killer executed in Arkansas in eight days as the state sought to carry out as many lethal injections as possible before one of its drugs expires Sunday.

An Associated Press reporter who witnessed the execution said that about three minutes in, Williams' body jerked 15 times in quick succession — lurching violently against the leather restraint across his chest — then the rate slowed for a final five movements.

J.R. Davis, a spokesman for Gov. Asa Hutchinson who did not witness the execution, called the movements "an involuntary muscular reaction" that he said was a widely known effect of the surgical sedative midazolam, the first of three drugs administered.

Williams' attorneys released a statement calling witness accounts "horrifying" and demanding an investigation into what they called the "problematic execution."

The American Civil Liberties Union of Arkansas backed up that call for a review, saying the state may have violated the Eighth Amendment's prohibition of cruel and unusual punishment. In a statement Friday, the organization's executive director Rita Sklar said the governor had "ignored the dangers ... all to beat the expiration date on a failed drug."

Arkansas had scheduled eight executions over an 11-day period before one of its lethal injection drugs expires April 30. That would have been the most in such a short time since the U.S. Supreme Court reinstated the death penalty in 1976, but courts issued stays for four of the inmates. The four lethal injections that were carried out included Monday's first double execution in the United States since 2000.