But the 39-page ruling from the appeal states: "The state courts' finding that Dassey's confession was voluntary was not beyond fair debate, but we conclude it was reasonable".
A federal judge had overturned Dassey's conviction previous year, accepting that detectives took advantage of Dassey's learning disabilities to get a confession.
Tear gas fired at anti-Trump protesters near Lebanon US embassy
In Beirut, Lebanese security forces clashed with protesters near the US Embassy on Sunday. Embassy with signs reading "Pray for Palestine" and "Freedom for Palestinians".
The narrow ruling has been slammed by legal professionals, including Judge Ilana Diamond Rovner.
Last year, the 28-year-old's conviction was overturned when a federal judge ruled the confession had been coerced.
'I view this as a profound miscarriage of justice'.
IDF Located, Neutralized New Terror Tunnel Crossing into Israel
Over the weekend, Israeli airstrikes hit several militant sites in Gaza in response to rocket fire, killing two Hamas militants. This was the second tunnel destroyed using a new system to detect, expose and destroy tunnels in the past six weeks.
Chief Judge Diane Wood, who disagreed, noted that Dassey was a low-functioning teenager with an IQ in the low 80s and without his confession, the state's case was "almost non-existent". Then, in June, a three-judge panel from the 7th U.S. Circuit Court of Appeals stood by the determination, but the state appealed for a full court ruling. However, his attorneys Laura Nirider and Robert Drizin have vowed to take his case to the US Supreme Court.
Avery's former lawyer Jerome Buting tweeted of the Dassey decision, "The Illusion of Justice continues, with the horrific decision of 4 judges on 7th Cir.to uphold Brendan Dassey's conviction". The appellate court rarely grants such reviews but opted to take Dassey's case in August without comment on the merits. But prosecutors appealed the decision, and the United States Court of Appeals, which blocked his release one day before he was set to be free, ordered him to remain in Columbia Correctional Institution in Portage, Wis., until his case was settled.
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.
IDF suspends commander for stealing fruit from Palestinian vendor during clashes
It said similar and simultaneous protests also were held Sunday in at least 10 provincial capitals and cities across Indonesia. The demonstrators set fire to Israeli and US flags, the news service added.
Authorities who worked on the cases say Making a Murderer was biased.