"The state courts' finding that Dassey's confession was voluntary was not beyond fair debate, but we conclude it was reasonable", the court's decision stated.
The judges were sharply divided, voting 4 to 3 that Dassey's confession wasn't coerced.
Dassey was sentenced to life in prison after confessing to police that he helped his uncle, Steven Avery, rape and kill Halbach, a freelance photographer, in 2005.
In a case featured in Netflix's documentary series "Making a Murderer", a federal court ruled Friday that Brendan Dassey's confession in the 2005 Wisconsin murder of Teresa Halbach was not coerced and denied him a new trial. A three-person panel of judges from the 7th Circuit Court of Appeals upheld that magistrate's decision this summer, but a larger gathering of the court's judges determined this week that Dassey's conviction should be upheld.
A federal appeals court on Friday upheld the murder conviction of a Wisconsin man in a case chronicled in the television documentary "Making of a Murderer", overturning a lower court judge who had tossed out the original guilty verdict.
Judge Ilana Diamond Rovner was one of the three to vote in Dassey's favor.
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However, the dissenting opinion from Chief Judge Diane Wood, who was joined by judges Ilena Rovner and Ann Williams, characterizes the majority's rejection of Dassey's petition for a new trial as a "travesty of justice".
Dassey's legal team might appeal Friday's decision, taking the fight to the Supreme Court. "His confession was not voluntary and his conviction should not stand, and yet an impaired teenager has been sentenced to life in prison".
Dassey's lawyers, Laura Nirider and Steven Drizin, issued a statement saying they were disappointed by the reversal and planned to appeal to the US Supreme Court. The appellate court rarely grants such reviews but opted to take Dassey's case in August without comment on the merits.
Brendan Dassey, now 28, is serving a life sentence in the Wisconsin prison system.
Both Avery and Dassey contend police framed them because they wanted revenge against Avery for filing a lawsuit against Manitowoc County over wrongful imprisonment for a sexual assault he didn't commit.