A judge in Wisconsin has overturned the conviction of Brendan Dassey in a case profiled in the Netflix documentary "Making A Murderer".

The District Court in Milwaukee overturned his initial conviction and ordered him to be freed within 90 days, unless the state decide to retry him.

Magistrate Judge William Duffin said investigators made false promises to Dassey by initially saying "he had nothing to worry about".

In Friday's ruling, Duffin said: "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".

Dassey was just 16-years-old when he confessed to helping his uncle, Steven Avery, rape and murder 25-year-old photographer Teresa Halbach who went to the Avery family salvage yard to photograph some vehicles.

Attorneys argued his constitutional rights had been violated throughout the investigation.

Avery was convicted in a separate trial and is serving a separate life sentence.

The case burst into the public's consciousness by featuring in the popular "Making A Murderer" series on Netflix.

Avery was initially sentenced to 18 years for sexual assault, before DNA evidence exonerated him in 2003.

He was then accused of Halbach's murder after taking out a lawsuit against Manitowoc County, the former district attorney and the county sheriff, for his previous conviction.