The decision from the US Seventh Circuit Court of Appeals reverses a federal judge's ruling and means Dassey's life sentence will remain in effect.
Dassey, now 28, was convicted in 2007 in the slaying of Wisconsin photographer Teresa Halbach. In 2005, Dassey, then 16, confessed to authorities that he assisted his uncle, Steven Avery, in raping and killing Halbach. Her charred remains were found on Avery family property in Manitowoc County.
Dassey later recanted his confession. His lawyers appealed through the state court system and then into the federal system, contending the confession was coerced. Court papers also raised the issue of Dassey's IQ, described as being in the "low to borderline" range.
On Friday, the full Seventh Circuit Court of Appeals ruled 4-3 that the confession should stand.
"Dassey spoke with the interrogators freely, after receiving and understanding Miranda warnings, and with his mother's consent," wrote Judge David Hamilton in the majority opinion.
"The interrogation took place in a comfortable setting, without any physical coercion or intimidation, without even raised voices, and over a relatively brief time. Dassey provided many of the most damning details himself in response to open-ended questions."
The three dissenting judges wrote that the decision was "a profound miscarriage of justice."
"What occurred here was the interrogation of an intellectually impaired juvenile," Judge Ilana Rovner wrote. "Dassey was subjected to myriad psychologically coercive techniques but the state court did not review his interrogation with the special care required by Supreme Court precedent. 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.
"Today's ruling contravenes a fundamental and time-honored position of the United States Supreme Court: interrogation tactics that may not be coercive when applied to adults are coercive when applied to children and the mentally impaired. Indeed, when such tactics are applied to vulnerable populations, the risk of false confession grows intolerably.
"Unfortunately, this time-worn lesson was ignored today by four judges in the case of Brendan Dassey. We at the Center on Wrongful Convictions of Youth are committed to continuing to fight on behalf of Brendan and others like him to prevent future miscarriages of justice."
US Magistrate Judge William Duffin had overturned Dassey's conviction
in August 2016, citing his age and lack of guardian during questioning by police.
State prosecutors appealed, but a three-judge panel for the Seventh Circuit Court of Appeals
affirmed Duffin's ruling last June by a 2-1 vote. Prosecutors appealed again, this time to the full appeals court. That led to Friday's ruling.
Wisconsin Attorney General Brad Schimel issued this statement Friday: "I'm gratified that the United States Court of Appeals for the Seventh Circuit reversed the district court's grant of habeas. Today's decision is a testament to the talent of the attorneys at the Wisconsin Department of Justice who have worked tirelessly to deliver justice for the family and friends of Teresa Halbach over the last decade."
Dassey has remained in prison
during the appeals process.
Avery, 54, is serving a life sentence at a Wisconsin prison. He maintained his innocence throughout his original trial. He said he was framed and is seeking a new trial
The 10-part Netflix series, which premiered in late 2015, renewed interest in Avery's ongoing legal troubles, leading to calls for his release.