A prisoner whose case was the subject of US documentary series Making a Murderer has had his conviction overturned.

Brendan Dassey confessed to helping his uncle, Steven Avery, carry out the rape and murder of 25-year-old photographer Teresa Halbach in 2005.

She was killed after she went to the Avery family car salvage yard to photograph some vehicles.

Dassey was arrested when he was 16, but his attorneys said his confession was coerced.

Federal judge William Duffin ruled on Friday that investigators had made false promises to Dassey that he "had nothing to worry about".

His ruling said the "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 US Constitution.

The US District Court in Milwaukee, Wisconsin, ordered that Dassey, now 26, should be freed within 90 days unless the case is appealed.

The state Department of Justice, which handled the case, did not immediately comment.

Avery was convicted separately in the homicide and is serving a life sentence. He is pursuing his own appeal.

The uncle - who maintains his innocence - had previously served 18 years for a rape he did not commit.

Making a Murderer cast doubt on the legal process used to convict Avery and Dassey, but prosecutors said the 10-hour series was biased.

Avery's lawyer, Kathleen Zellner, said she visited him in prison on Friday and he told her he was "so happy" his nephew had been exonerated.

Ms Zellner said she is confident Avery's conviction will eventually also be overturned "when an unbiased court" reviews it.