POTOSI, Mo. (AP) _ A man convicted of robbing a teen-age gas station clerk and shooting him in the back of the head as he lay on the ground was executed early Wednesday.

Stephen K. Johns, 55, was pronounced dead at 12:03 a.m. at the Potosi Correctional Center, two minutes after the first of three lethal doses was administered, state Department of Corrections spokesman John Fougere said.

His fate was sealed late Tuesday when the Missouri Supreme Court, the U.S. Supreme Court and Gov. Bob Holden refused to halt the execution, Missouri's seventh this year and the state's 53rd since it resumed capital punishment in 1989.

A jury in St. Louis found Johns guilty of shooting 17-year-old Don D. Voepel three times in the back of the head at close range after a robbery that netted $248. Voepel wasn't even supposed to work the night of Feb. 18, 1982; a co-worker had a date and Voepel agreed to cover his shift.

In an interview Tuesday, Johns continued to maintain his innocence, insisting he has never been to the gas station where Voepel was killed.

``I didn't even know it was there,'' Johns said.

He had been on death row since being sentenced in January 1983.