The US state of Wisconsin has appealed against a federal judge’s ruling to overturn the conviction of Brendan Dassey.

In 2007 Dassey and his uncle Steven Avery, featured in the Netflix series Making a Murderer, were  sentenced to life in prison for the murder of photographer Teresa Halbach.

The charred remains of Halbach, 25, were discovered in a burn pit on Avery’s rural scrap car property in Manitowoc County.

Three weeks ago, Dassey had his conviction overturned by a federal judge in Milwaukee.

Teresa Halbach’s family wants justice. Photo: Netflix

In a 91-page decision, Judge William E. Duffin said investigators repeatedly made false promises to Dassey, who at the time was 16, in gaining a confession.

The judge found the confession was involuntary.

Dassey, said to have a low IQ, was questioned three times with no legal representation or adult present.

Dassey initially confessed but then later recanted his confession.

Wisconsin Attorney-General Brad Schimel has now appealed against Judge Duffin’s ruling.

Mr Schimel said two state courts “properly concluded that Dassey’s confession was voluntary, and the investigators did not use constitutionally impermissible tactics”.

“The Halbach family has been notified of the appeal and fully supports the state’s decision to seek justice on behalf of their daughter,” Mr Schimel said.

Following the release of the Netflix series, fans called for Dassey and Avery to be freed. Almost 130,000 people signed a White House petition asking for a presidential pardon.

Avery’s attorney has filed a motion for permission to perform tests on evidence from the case.

Avery’s attorney wants new tests on evidence. Photo: Netflix

Attorney Kathleen Zellner requested advanced forensic testing be conducted, alleging it will show her client was framed for the murder of Halbach.

Zellner also filed a motion asking the appeals court to pause on the proceedings until testing was completed.

“Mr Avery is requesting, and is willing to pay for, the most comprehensive, thorough, and advanced forensic testing ever requested by a criminal defendant in the State of Wisconsin,” Zellner said.

Zellner believes new tests will reveal Avery isn’t guilty and that someone else killed Halbach.

Zellner has requested testing and retesting on evidence, including Halbach’s vehicle key, which was found in Avery’s room with his DNA on it; Avery’s blood found in the vehicle; and a pair of women’s underwear found in the yard to see if they belonged to Halbach and contain male DNA.

“The most reassuring thing is that we are going to get to the bottom of who killed Teresa Halbach,” Zellner told the New York Post.

“And we firmly believe that we will establish it was not Steven Avery.”

Avery, 54, was charged with sexually assaulting and killing Halbach, who disappeared after travelling to the salvage yard to shoot photos for a car magazine.

Avery and his nephew, Dassey, lived on the property.

Dassey was convicted of being party to first-degree intentional homicide, mutilating a corpse and sexual assault and was sentenced to life.