Updated

The last of four Arkansas executions over an eight-day period has prompted calls for investigations after the inmate lurched and convulsed while strapped to the gurney.

An Associated Press reporter who witnessed Kenneth Williams' execution Thursday said that about three minutes into the lethal injection, his body jerked 15 times in quick succession. He lurched violently against the leather chest restraint, then the rate slowed for a final five movements.

One of Williams' attorneys called the execution "horrifying." A spokesman for Gov. Asa Hutchinson called it "an involuntary muscular reaction."

The compressed lethal injection timeline could attract more scrutiny after Williams' death. Arkansas sought to carry out as many lethal injections as possible before one of its drugs expires Sunday. It executed four prisoners, while four others received court stays.