A 16-year-old boy pleaded not guilty Friday to 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 the teen had been threatened by his uncle, 43-year-old Steven Avery, who is also accused in the Oct. 31 slaying.

"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 fire pit there.

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

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 was in special classes after being tested as "significantly below average" in intelligence, Sczygelski said.

Special prosecutor Ken Kratz rebutted the characterization of the boy as a victim.

"There is only one victim in this case: Miss Halbach," he said.

Dassey 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.

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

Sczygelski will withdraw as Dassey's attorney after learning later Friday that they are distantly related, he said.

Kratz said Avery gave a fake name when he called Auto Trader Magazine to request a visit from Halbach, a freelancer. Halbach had been there before for similar appointments.

Dassey said he got involved only because he went to Avery's trailer to deliver some mail, 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.

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.