Arkansas executes 4th prisoner in 8 days as lethal injection drug nears expiration

— -- Arkansas executed its fourth prisoner in eight days on Thursday night, within an hour of the U.S. Supreme Court denying a motion for a stay of execution.

Kenneth Williams, a 38-year-old man convicted of two murders, was scheduled to be put to death by lethal injection at 7 p.m. local time at a correctional facility in Varmer, but the execution was delayed so that the Supreme Court could resolve a handful of other cases before considering Williams' fate.

Attorneys for Williams filed a motion Thursday evening to the country's highest court after a federal appeals court denied his request to stop the execution, the Associated Press reported. But within three hours, the Supreme Court denied all motions for a stay.

The execution comes as one of the trio of drugs it uses in lethal injections is due to expire at the end of the month.

It is not known how Arkansas will carry out future executions after the drug expires.

Williams was serving life in prison for the murder of 19-year-old Dominique Hurd when he escaped in 1999 and killed Cecil Boren. His capture resulted in another man's death, Michael Greenwood, who was killed in a vehicle crash with Williams.

"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," said Arkansas Gov. Asa Hutchinson in a statement. "Carrying out the penalty of the jury in the Kenneth Williams case was necessary. There has never been a question of guilt."

Arkansas initially planned to execute eight convicts over a span of 11 days as it rushes to use the state's supply of a key lethal injection drug. Four of the planned executions were delayed after legal challenges.

Before last week, the state had not executed anyone since 2005. Prison officials carried out the first of the scheduled executions last Thursday and put two more to death in a rare double execution, the first in the U.S. since 2000, on Monday.

Arkansas' supply of midazolam, one of three drugs in a lethal cocktail, expires on April 30 and the state said it has no source for additional doses.

Prior to the executions, hundreds of death penalty opponents, including actor Johnny Depp and former Arkansas death row inmate Damien Echols -- who was released on DNA evidence after spending 18 years in prison -- held a rally in Little Rock, Arkansas calling for the state to end capital punishment.

ABC News' Darren Reynolds and Karma Allen contributed to this report.The Associated Press also contributed to this report.