The execution is the 22nd in the U.S. this year and the sixth in Alabama, which has put three of the men to death using nitrogen gas.

An Alabama prisoner convicted of the 1994 murder of a female hitchhiker is slated to become the third person executed by nitrogen gas.

Prison officials in Alabama have executed convicted murderer Carey Dale Grayson, marking the state's sixth death sentence and third using the nitrogen gas method so far this year.