Brendan Dassey was sentenced to life in prison in 2007 in photographer Teresa Halbach's death two years earlier. Dassey told detectives he helped his uncle, Steven Avery, rape and kill Halbach in the Avery family's Manitowoc County salvage yard.
A federal magistrate judge ruled in August that investigators coerced Dassey, who was 16 at the time and suffered from cognitive problems, into confessing.
The state Justice Department appealed the ruling to the 7th U.S. Circuit Court of Appeals. A three-judge panel from the 7th Circuit on Thursday affirmed the lower court and said Dassey should be freed unless the state chooses to retry him.
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