A federal appeals has upheld the district court's decision to overturn Brendan Dassey's conviction.
The writ of habeas corpus has been granted unless the state decides to retry Dassey within 90 days of Thursday.
A federal court in Wisconsin overturned Dassey's conviction in August of 2016.
Magistrate Judge William Duffin said in the ruling that investigators made false promises to Dassey by assuring him "he had nothing to worry about."
"These repeated false promises, when considered in conjunction with all relevant factors, most especially Dassey's age, intellectual deficits, and the absence of a supportive adult, rendered Dassey's confession involuntary under the Fifth and Fourteenth Amendments," Duffin wrote.
The ruling came after Dassey's appeal was rejected by state courts.
Dassey was serving a life sentence for helping his uncle, Steven Avery, murder Teresa Halbach. The case has gained international attention due to the Netflix documentary "Making a Murderer."