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.

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.