ATLANTA - Mark Anthony Duke's death sentence was already reduced to life in prison without parole for the slayings of four people. Now he's asking a federal appeals court to overturn his conviction because he claims Alabama prosecutors made a crucial error during his trial.

Duke and another man were sentenced to death for the 1997 fatal shootings of Duke's father and his father's fiance and for cutting the throats of her two young daughters when they ran out of bullets. But Duke's sentence was reduced to life after the Supreme Court ruled that Duke couldn't be executed because he was 16 at the time of the killings.

On Friday, Duke's attorneys asked the 11th Circuit Court of Appeals to toss the conviction because they say a prosecutor wrongly encouraged a jury to question his decision to stay silent at trial. They said that would violate federal case law that dates back to 1965.

But Alabama state attorneys countered that the statement was an isolated comment that didn't impact the outcome of the case. Tom Leverette, an Alabama assistant attorney general, said the "overwhelming evidence here at best makes this a harmless error case."

On Friday, the three-judge panel seemed skeptical of Duke's challenge.

U.S. Circuit Judge J. L. Edmondson said there's no record that indicates the prosecutor ever actually pointed directly at Duke. The mere assertion by Duke's lawyer isn't enough, he said, or else "I don't understand why I can't just say the Green Man walked into the courtroom ... and it would be part of the record."

Leverette told the panel, which didn't immediately issue a ruling, that the statement wasn't referring to Duke. He also said that it didn't matter either way because the jury heard enough other evidence to justify his conviction.

"In the heat of battle, in closing arguments, attorneys may not say something exactly the way they want to say it," said Leverette. "But they get the meaning across."

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