Two Illinois parents who were charged with killing their 5-year-old son after reporting him missing last week allegedly abused him so badly it killed him, according to prosecutors.

Andrew "AJ" Freund's body was found wrapped in plastic and buried in a shallow grave near his home in Crystal Lake, Illinois, on Wednesday. Details of the abuse the boy allegedly endured at the hands of his parents, Joann Cunningham, 36, and Andrew Freund Sr., 60 was released in a criminal complaint Thursday

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Court documents allege that he died after he was beaten and forced to stand in a cold shower for a long period of time. Freund was reported missing on April 18 — after his parents said they hadn't seen him since the night before — but investigators believe he was killed on April 15.

The McHenry County Coroner's Office announced Freund's cause of death on Thursday as "craniocerebral trauma as a consequence of multiple blunt force injuries."

The boy's family had a history with local police and the Illinois Department of Children and Family Service. The agency launched an investigation shortly after Freund was born when tests revealed there were opiates in his body, which prompted officials to place the boy in foster care for nearly two years.

The Crystal Lake Police Department had repeatedly been called to the Freund family home, where officers described horrid living conditions, including broken windows, evidence of drug use and the overpowering smell of dog feces.

"It is my hope that you may have some solace in knowing that AJ is no longer suffering and his killers have been brought to justice," Crystal Lake Police Chief James Black said in a statement posted to Facebook on Wednesday, directed at Freund's family. "To AJ, we know you are at peace playing in heaven’s playground and are happy you no longer have to suffer."

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Both Cunningham and Freund Sr. appeared in court on Thursday, where they were represented by lawyers from the McHenry County public defender's office. They have been charged with first-degree murder, aggravated battery and several other charges.

A judge set each of their bail at $5 million, meaning each parent would have to post 10%, or $500,000, to be released from jail. The two were ordered to return to court on Monday for a preliminary hearing.

The Associated Press contributed to this report.