Updated

Arkansas has put to death its fourth inmate in eight days, in what is now the most aggressive execution schedule since the death penalty was reinstated in 1976.

Kenneth Williams, 38, received a lethal injection Thursday night at the Cummins Unit prison at Varner for the death of a former deputy warden killed after Williams escaped from prison in 1999; the convicted killer was only three weeks into a life sentence for the death of college cheerleader when he escaped.

The aggressive execution schedule comes as one of the surgical sedatives used during lethal injections is set to expire 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.

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.

Williams was sentenced to death for killing a former deputy warden 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.

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.

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 said.

"After the state injects Mr. Williams with vecuronium bromide ... most or all of the manifestations of his extreme pain and suffering will not be discernible to witnesses," they wrote to the Arkansas Supreme Court, which rejected his request to stop the execution.

The Associated Press contributed to this report