Difficulty in setting up the lethal injection caused a delay of more than an hour in Tuesday's execution of a man convicted of killing a gas station clerk during a series of robberies.

Joseph Lewis Clark, 57, died at 11:26 a.m. at the Southern Ohio Correctional Facility. He was sentenced to die in November 1984 for killing David Manning.

The execution was set to begin at 10 a.m., but was delayed as the team worked to find a vein in his right arm to administer the injection. Clark said "they're not working" as the team tried to start the injection.

After 25 minutes of trying to find a vein, a curtain was pulled over the window separating witnesses from the area where the execution was to take place. Clark was heard moaning and groaning and when the curtain was reopened he had a shunt in one arm.

Prisons director Terry Collins said a vein in Clark's arm had collapsed and that Clark's history of drug use could have been a factor.

"The team here is a very professional team. They're doing a very, very difficult job under difficult circumstances," Collins said.

Clark is the 21st man executed since Ohio resumed capital punishment in 1999.

He had confessed that he killed Manning, saying he was trying to get money for drugs. He also was sentenced to life in prison for killing a store clerk the day before Manning's death as part of the series of robberies that lasted over a week.

Gov. Bob Taft rejected Clark's appeal for clemency last week.