A woman charged with murdering and dismembering her adopted daughter in a shared rape and murder fantasy with her boyfriend was a child welfare worker, it has been revealed.

Sara Packer, 41, allegedly watched as 14-year-old Grace Packer was raped by Jacob Sullivan, beaten and strangled. Grace's body was later dumped in a forest in Pennsylvania.

Mr Sullivan, 44, was arrested by Bucks County detectives and formally charged with rape and murder on Sunday morning over the death, while Ms Packer was arraigned on charges including criminal homicide, rape and abuse of a corpse.

Now authorities have said Ms Packer worked as an adoption supervisor for seven years at the Northampton County Children, Youth and Families Division, WKBN said.

Court documents claim the couple began plotting to kill the girl in the autumn of 2015. Mr Sullivan told detectives that he and Sara Packer drove Grace to their new home in Quakertown on July 8, 2016.

Once inside, Mr Sullivan allegedly struck the teenager in the face and then raped her in the attic as her adoptive mother watched. Mr Sullivan allegedly told detectives he had raped Grace previously and that the girl's adopted mother enjoyed watching.

They forced her to take an overdose of over-the-counter drugs, assuming that would kill her, court documents said. But finding her alive when they returned many hours later, Mr Sullivan strangled her to death, he allegedly told detectives.

The pair kept the body in the attic for nearly four months, covering it with cat litter to mask the smell, authorities said.

Ahead of their court appearance, Mr Sullivan allegedly said: "I'm sorry for what I did."