MANITOWOC, Wis. (AP) — A 16-year-old boy pleaded not guilty Friday to charges of helping his uncle sexually assault and kill a photographer, then burning her body and cleaning up the blood.

Brendan Dassey's attorney told the court that the teen had been threatened by his uncle, 43-year-old Steven Avery, who is also accused in the Halloween murder.

"He essentially has been victimized by Mr. Avery as well," Ralph Sczygelski said.

Prosecutors say the two killed 25-year-old Teresa Halbach after Avery called her to the family's salvage lot to photograph a minivan that was for sale. Her remains were found in a burn pit there.

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According to a complaint filed Thursday, Dassey told investigators that Halbach was shackled naked to a bed, begging for help as Avery stabbed her and Dassey slit her throat. The two then hauled Halbach's body to a garage, shot her and burned her corpse in a pit, piling on tires, brush and debris to fuel the blaze, the complaint said.

Dassey's mother, Barb Janda, told the judge Friday that her son was forced to do the crime. "He didn't do this on his own," she said.

Dassey, who appeared Friday in handcuffs and faded green jail clothes, was charged with being party to murder, which carries a mandatory life prison term, as well as mutilation of a corpse and first-degree sexual assault.

Steven Avery

He waived his right to a preliminary hearing, and Judge Jerome Fox set bail at $250,000 cash.

Special prosecutor Ken Kratz said Avery gave a fake name when he called Auto Trader Magazine to request a visit from Halbach, one of the magazine's freelancers. Halbach had been there several times before for similar appointments.

Dassey said he got involved in the killing only because he went to Avery's trailer to deliver some mail after school, the complaint said.

Besides the murder charge, Avery faces charges of mutilating a corpse and possession of firearms by a felon, and Kratz said additional charges will be sought based on Dassey's statement.

"There is a substantial amount of physical evidence that now makes sense, fits a lot of pieces together," Kratz told a news conference Thursday.

Avery, who remains jailed in lieu of a $500,000 bond, denies knowledge of Halbach's death.

He was released from prison in 2003 after serving 18 years for rape before being exonerated by DNA evidence.