Dassey, now 27, is serving a life sentence for the death of 25-year-old Teresa Halbach, who disappeared on Halloween in 2005 after she went to take photos for an auto magazine at a salvage yard owned by Dassey's uncle, Steven Avery, in Manitowoc County, Wis. Avery was tried separately and is serving a life sentence without the possibility of parole.
The 2-to-1 decision upholds a federal judge's ruling last August.
Duffin ruled that authorities had inappropriately promised the then-16-year-old leniency in exchange for his confession. However, Wisconsin filed an appeal on the ruling and Dassey was kept under lock and key while the appeals court heard the case.
It's not immediately clear if Dassey's release will have any impact on Avery's case, though his lawyer, Kathleen Zellner, has filed a 1,272-page motion requesting a new trial.
Dassey's attorneys said the ruling took their client one step closer to freedom.
However, Johnny Koremenos, a spokesman for Wisconsin Attorney General Brad Schimel, said it seemed likely they would appeal it once more. "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".More news: Defiant Nancy Pelosi says she's not going anywhere
In addition to providing evidence that Dassey was coerced into a confession, "Making a Murderer", a 10-part series by Laura Ricciardi and Moira Demos released in December 2015, suggested that Dassey was improperly questioned by police investigators in 2006, when he was 16, and that he was mentally unfit. During two separate police interrogations, he confessed to helping Avery, his uncle, rape and kill Halbach, 25, who went missing on October 31, 2005.
Circuit Judges Ilana Diamond Rovner and Ann Claire Williams were the majority.
In the dissent, Judge David F. Hamilton said, "We also should not lose sight of the most damning physical evidence: the bones of Teresa Halbach, broken and charred, buried in the ashes of Avery's burn pit".
"In evaluating whether a confession is voluntary, courts must engage in the kind of searching and analysis that the federal courts performed, that the state courts did not".
Zellner visited Avery at the Waupun Correctional Institution Friday morning, a day after the ruling. In November of past year, a Milwaukee judge finally recognized that and made a decision to overturn Dassey's conviction, granting him supervised release from prison while the state decided it it wanted to pursue a retrial.
Calumet County Prosecutor Ken Kratz called a news conference shortly after investigators secured the confession, saying that Dassey described in detail Halbach's brutal assault and slaying.