The court has ordered a new trial on the penalty phase.
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Laci disappeared from their Modesto home on Christmas Eve in 2002 while eight months pregnant. Investigators said Peterson dumped the bodies from his fishing boat into San Francisco Bay, where they surfaced months later.
"Peterson contends his trial was flawed for multiple reasons, beginning with the unusual amount of pretrial publicity that surrounded the case. We reject Peterson's claim that he received an unfair trial as to guilt and thus affirm his convictions for murder," the court said.
But it said the trial judge "made a series of clear and significant errors in jury selection that, under long-standing United States Supreme Court precedent, undermined Peterson's right to an impartial jury at the penalty phase."
It agreed with his argument that potential jurors were improperly dismissed from the jury pool after saying they personally disagreed with the death penalty but would be willing to follow the law and impose it.
Peterson, who is now 47, contended on appeal that he couldn't get a fair trial because of the massive publicity that followed, although the proceedings were moved nearly 90 miles away from his Central Valley home of Modesto to San Mateo County, south of San Francisco.
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Peterson was convicted by a jury in 2004 and in 2005 was sentenced to death.
The Associated Press contributed to this report.