By Steve Olafson
OKLAHOMA CITY (Reuters) - A convicted Oklahoma killer who was spared execution once but asked for a new trial and was sentenced to death a second time was executed by lethal injection on Tuesday.
Michael B. Selsor, 57, was the 18th person executed in the United States this year and the third person executed in Oklahoma in 2012. He was pronounced dead at 6:06 p.m. local time (1106 GMT) at the state prison in McAlester, Oklahoma, a prison spokesman said.
He was executed for killing convenience store clerk Clayton Chandler on September 22, 1975, during an armed robbery in Tulsa, Oklahoma.
Selsor was sentenced to death in 1976, but later that year the U.S. Supreme Court ruled Oklahoma's capital punishment law was unconstitutional and his sentence was modified to life in prison.
With Oklahoma's death penalty briefly in limbo, the state's criminal appeals court decided in 1976 that former death row inmates whose sentences were modified would not face the death penalty again should they win new trials. The death penalty in Oklahoma was restored in 1977.
Selsor sought a new trial and received one in 1998. But again he was found guilty and sentenced to death.
The Oklahoma Court of Criminal Appeals in 1997 overruled one of its earlier rulings affecting inmates like Selsor, opening the death penalty option at his new trial, according to one of his lawyers Gary Peterson.
Selsor immediately appealed his second death sentence but was told he had no right to be warned that Oklahoma law could be changed to make him eligible for the death penalty at his second trial, Peterson said.
"It's a tragic story," Peterson said. "He just had the legal rug pulled out from under him."
Before his execution, Selsor expressed love for his son and sister, and implored a friend "to keep up the struggle."
"I'll be waiting at the gates of heaven for you. I hope the rest of you make it there as well. I'm ready," Selsor was quoted by the spokesman as saying.
(Editing by Dan Burns, Greg McCune and Lisa Shumaker)