A judge on Friday overturned the conviction of a Wisconsin man found guilty of helping his uncle kill a woman in a case profiled in the Netflix documentary series Making a Murderer, ruling that investigators coerced a confession using deceptive tactics.

U.S. Magistrate William Duffin in Milwaukee ordered Brendan Dassey freed within 90 days unless prosecutors decide to retry him. The state Department of Justice, which handled the case, declined to comment Friday.

Dassey's case burst into the public's consciousness with the popularity of the Making a Murderer series that debuted in December. The filmmakers cast doubt on the legal process used to convict Dassey and his uncle Steven Avery in the death of Teresa Halbach, and their work sparked national interest and conjecture. Authorities involved in the case have called the 10-hour series biased, while the filmmakers have stood by their work.

Dassey confessed to helping Avery carry out the rape and killing of Halbach, but his attorneys argued that his constitutional rights were violated throughout the investigation. Dassey's confession wasn't presented as evidence in Avery's trial. Both men are serving life sentences.

Duffin said in his 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 (of the U.S. Constitution)," Duffin wrote.

Co-defendants tried separately

Dassey was 16 when Halbach, a photographer, was killed in 2005 after she went to the Avery family auto salvage yard to take pictures of some vehicles. Court papers describe Dassey as a slow learner with poor grades, with difficulty understanding some aspects of language and expressing himself verbally. He was also described as extremely introverted and poor at picking up nonverbal communications such as body language and tone.

Dassey, now 26, was convicted of first-degree intentional homicide, second-degree sexual assault, and mutilation of a corpse in Halbach's killing. Avery was tried and convicted separately in the homicide.

Avery made headlines in 2003 when he was released from prison after spending 18 years behind bars for a rape he didn't commit. After being freed, he had a $36 million US lawsuit pending against public officials when Halbach disappeared on Halloween 2005.

Steven Avery is seen in a still image made from video from part of the true crime series Making a Murderer in a handout image. Avery has launched an appeal of his conviction. (Netflix/The Canadian Press)

Friday's ruling came after Dassey's appeal was rejected by state courts. The judge said that Dassey's confession to police in 2006 was "so clearly involuntary" that a state appeals court ruling to the contrary was an unreasonable application of established federal law.

"The court does not reach this conclusion lightly," Duffin wrote.

The investigators did not have any ill motive, the judge wrote, but rather "an intentional and concerted effort to trick Dassey into confessing."

The error was not harmless because Dassey's confession was the entirety of the case against him, the judge said.

Attorneys for Dassey did not immediately return messages seeking comment. A brother who has acted as a Halbach family spokesman did not immediately respond to phone messages and an email.

Kathleen Zellner, an attorney for Avery, said in a statement that Avery was thrilled for his nephew. Avery is pursuing his own appeal.

"We know when an unbiased court reviews all of the new evidence we have, Steven will have his conviction overturned as well," Zellner said.

Joe Friedberg, a defence attorney in Minnesota who was not involved in the case but is familiar with it and participated in a forum on it with Avery's first defence attorney, said he doesn't believe the decision will have any bearing on Avery's case.

"The kid's confession was not entered into evidence against Avery, and I don't think it impacted Avery's trial at all," Friedberg said.

Netflix last month announced that new episodes of Making a Murderer were in production to follow appeals by both Avery and Dassey.

"As we have done for the past 10 years, we will continue to document the story as it unfolds, and follow it wherever it may lead," filmmakers Laura Ricciardi and Moira Demos said in a written statement following Friday's ruling.