The Seventh Circuit Court of Appeals in Chicago met in September to consider whether Brendan Dassey, now 28 but 16 at the time of his conviction, was properly interviewed by police or forced into confessing.
In the end, by a vote of 4-3, the judges ruled that the confession was not forced.
It was a key element in Dassey's 2007 conviction for the murder of Teresa Halbach in Wisconsin. Dassey was accused of aiding his uncle Steven Avery in the murder, and they were both given life sentences.
A federal court in Wisconsin last year found that Dassey, over the course of multiple interviews, had been tricked into confessing -- both by police and an unsympathetic private investigator working for his defense attorney.
The judge pointed out in his ruling that Dassey had a low IQ, was easily coerced and changed his story multiple times, and was without proper legal representation during his police interrogations.
A three-judge panel of the federal appeals court in Chicago agreed in a 2-1 ruling in June and ordered Dassey freed.
But the state appealed, asking the full panel of the appeals court to reconsider the case, claiming the earlier decisions had "rewritten the rules for juvenile interrogations."
Halbach's murder was featured in the documentary series "Making a Murderer" launched in December 2015.
It raised questions about the US legal system and prompted many viewers to believe Dassey and Avery were wrongly convicted.