A mother and her boyfriend allegedly beat, poisoned, bound and gagged an adopted teenage girl in a sick "rape-murder fantasy" shared by the couple.
Grace Packer's dismembered remains were found among woods in Bear Creek Township, Pennsylvania on Halloween, one month after she disappeared.
Sara Packer, 41, and Jacob Sullivan, 44, are facing charges of criminal homicide, rape, kidnapping and abuse of a corpse over the death of Grace, online court documents show.
Prosecutors said the couple shared a "rape-murder fantasy" and had planned the attack for more than a year, Philadelphia Inquirer reported.
"The hours and the days leading up to her murder were probably the most horrible and traumatic that any person should ever have had to experience," Bucks County District Attorney Matthew Weintraub said.
The boyfriend reportedly confessed to police they were both responsible for Grace's murder while in hospital after a failed suicide attempt.
On July 8, Grace, who had been adopted by Ms Packer, was taken to the attic of a house the couple rented.
She was then beaten and raped by Mr Sullivan as her mother watched, an affidavit read.
Weintraub said the teen was then poisoned, bound and gagged and left for dead.
When the couple returned hours later, they discovered Grace was still alive, so Mr Sullivan reportedly choked her to death.
The mother's boyfriend told police it took much longer than he expected to strangle her, the affidavit said.
Investigators said the couple packed Grace's body with cat litter to mask decomposition smells and stored it in the attic.
It is alleged the couple dismembered the body in October after being scared by a police visit.
The girl's torso was later found by two hunters in a Luzerne County park and K-9 teams found the legs and arms nearby, a criminal complaint said.
The couple didn't enter pleas, were both denied bail and are expected to be transferred to Bucks County Prison.
Mr Sullivan apologised as he was led into court, the Intelligencer newspaper reported.
"I'm sorry for what I did," he said. "It was wrong."
Ms Packer had previously been charged with child endangerment and obstruction.
Police said she provided misleading information to investigators when reporting that her adopted daughter was missing from their Abington home in July.
After numerous unsuccessful attempts to contact Ms Packer in August, investigators learned the following month that Grace and her younger brother had been withdrawn from Abington School District.
It was later revealed, Ms Packer had moved to Quakertown without telling police, and her 12-year-old adopted son, Grace's biological brother, had been enrolled in Quakertown School District.
Authorities said the mother kept cashing her daughter's monthly $700 Social Security checks and updated disability forms in August without mentioning the girl's disappearance.
It's unclear what the girl's disability was.
Mr Sullivan and Ms Packer made a suicide pact and tried to overdose on prescription pills on December 30, a criminal complaint said.
They were hospitalised after being found by a woman who lived with them.