VARNER, ARK. — Arkansas executed its fourth inmate in eight days Thursday night, wrapping up an accelerated schedule with a lethal injection that left the prisoner lurching and convulsing 20 times before he died.



Kenneth Williams, 38, was pronounced dead at 11:05 p.m., 13 minutes after the execution began at the Cummins Unit prison at Varner.



An Associated Press reporter who witnessed the lethal injection said Williams' body jerked 15 times in quick succession, 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 it "an involuntary muscular reaction" that he said was a widely known effect of the sedative midazolam, the first of three drugs administered.



Williams' attorneys are calling for an investigation into the execution.



Arkansas had scheduled eight executions over an 11-day period before one of its lethal injection drugs expires on Sunday. That would have been the most in such a compressed period 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.



"I extend my sincerest of apologies to the families I have senselessly wronged and deprived of their loved ones," Williams said in a final statement he read from the death chamber. "... I was more than wrong. The crimes I perpetrated against you all was senseless, extremely hurtful and inexcusable."



Williams also spoke in tongues, the unintelligible but language-like speech used in some religions. But his prayer faded off as the sedative midazolam took effect. His final words were, "The words that I speak will forever be, will forever ..." before he fell silent.



The inmate breathed heavily through his nose until just after three minutes into his execution, when his chest leaped forward in a series of what seemed like involuntary movements. His right hand never clenched and his face remained what one media witness called "serene."



After the jerking, Williams breathed through his mouth and moaned or groaned once — during a consciousness check — until falling still seven minutes into the lethal injection.



Williams was sentenced to death for killing a former deputy warden, Cecil Boren, after he escaped from prison in 1999. At the time of his escape in a 500-gallon barrel of hog slop, Williams was less than three weeks into a life term for the death of a college cheerleader.



"Any amount of movement he might have had was far less than any of his victims," said Jodie Efird, one of Boren's daughters, who witnessed the execution.



State officials have declared the string of executions a success, using terms like "closure" for the victims' families. The inmates have died within 20 minutes of their executions beginning, a contrast from midazolam-related executions in other states that took anywhere from 43 minutes to two hours. The inmates' lawyers have said there are still flaws and that there is no certainty that the inmates aren't suffering while they die.



"The long path of justice ended tonight and Arkansans can reflect on the last two weeks with confidence that our system of laws in this state has worked," Hutchinson said in a statement issued after the execution. "Carrying out the penalty of the jury in the Kenneth Williams case was necessary. There has never been a question of guilt."



Arkansas scheduled the executions for the final two weeks of April because its supply of midazolam, normally a surgical sedative, expires on Sunday. The Arkansas Department of Correction has said it has no new source for the drug — though it has made similar remarks previously yet still found a new stash.



Williams' lawyers said he had sickle cell trait, lupus and brain damage, and argued the combined maladies could subject him to an exceptionally painful execution in violation of the U.S. Constitution. Arkansas' "one size fits all" execution protocol could leave him in pain after a paralytic agent renders him unable to move, they'd argued to state and federal courts, which all rejected his claims.



One of Williams' attorneys, Shawn Nolan, described the accounts of Williams' execution as "horrifying."



"We tried over and over again to get the state to comport with their own protocol to avoid torturing our client to death, and yet reports from the execution witnesses indicate that Mr. Williams suffered during this execution," Nolan said.



Williams was sentenced to death for killing Boren after escaping from the Cummins Unit prison in a barrel holding a mishmash of kitchen scraps. He left the prison — where the execution chamber is located in another part of the facility — less than three weeks into a life prison term for killing University of Arkansas at Pine Bluff cheerleader Dominique Hurd in 1998. At the conclusion of that trial, he had taunted the young woman's family by turning to them after the sentence was announced and saying "You thought I was going to die, didn't you?"



After jumping from the barrel, he sneaked along a tree line until reaching Boren's house. He killed Boren, stole guns and Boren's truck and then drove away to Missouri. There, he crashed into a water-delivery truck, killing the driver. While in prison, he confessed to killing another person in 1998.



At the time of Boren's death, investigators said it did not appear Boren was targeted because of his former employment by the Arkansas Department of Correction.