Brendan Dassey, now 27, didn't admit "from his own free will" that he helped his uncle murder and sexually assault photographer Teresa Halbach in 2005, a US 7th Circuit Court of Appeals panel ruled in a 2-1 vote.
Dassey, then just 16 years old, confessed to helping his uncle Avery - who had already served 18 years for a crime he did not commit - rape, kill and mutilate Ms Halbach. In November, the state Justice Department appealed Duffin's decision to the US Court of Appeals for the Seventh Circuit - blocking Dassey's release pending a hearing.
The state Justice Department appealed the ruling to the 7th U.S. Circuit Court of Appeals, a move that kept Dassey, now 27, behind bars pending the outcome.
"We are evaluating the 2-1 decision from the court. The pattern of questions demonstrates that the message the investigators conveyed is that the "truth" was what they wanted to hear", reads the decision. The ruling came 10 years after Dassey was convicted of sexually assaulting and killing a 25-year-old photographer. Dassey told detectives he helped his uncle, Steven Avery, rape and kill Halbach. They have 90 days to decide whether they'll retry him.
He added, "We continue to send our condolences to the Halbach family as they have to suffer through another attempt by Mr. Dassey to re-litigate his guilty verdict and sentence".
Dassey offered conflicting versions of his story, according to the court opinion.