(CNN) -- A fourth suspect is in custody in the alleged forced labor of a mentally disabled woman held with her young daughter, according to federal prosecutors in Ohio.
Dezerah Silsby, 21, turned herself in and is scheduled to appear in court Thursday afternoon, authorities said.
The two victims were held in an apartment crowded with people and animals for more than a year, and the woman was forced to perform manual labor and threatened with dogs and snakes to keep them compliant, authorities have said.
Federal prosecutors said the people accused of holding the pair in Ashland, about 60 miles south of Cleveland, collected the woman's government benefits and beat her in order to get painkillers for themselves.
They kept her in a room with a free-ranging iguana and ordered her to feed the reptile fruits and vegetables her daughter was denied, according to court papers.
"The living conditions were simply subhuman," said Steven Dettelbach, U.S. attorney for the Northern District of Ohio.
The mother and daughter were sometimes forced to eat dog food, according to a law enforcement source with firsthand knowledge of the investigation.
They were also frequently denied access to the bathroom, FBI Special Agent Eric Smith told reporters on Tuesday.
"They were physically punished for toiletry accidents," he said, "and they were threatened not only with weapons but also with vicious animals, to include pit bulls and pythons."
Officials earlier said 26-year-old Jordie Callahan, 31-year-old Jessica Hunt and 33-year-old Daniel Brown were arrested and charged with forced labor. Callahan is facing an additional count of witness tampering, and another arrest could come soon, the U.S. attorney's office in Cleveland said.
Callahan's mother, Becky, told CNN's Piers Morgan that the accusations are false, and that the alleged victim was allowed to leave the apartment whenever she wanted.
"There are so many lies going on," she said, accusing investigators of trumping up the case to draw attention to the small city of Ashland.
"She was giving them a couple hundred dollars a month for staying there. She was getting her own food. She wasn't being starved," Callahan's mother said.