(CNN) -- A fourth suspect is in custody in the alleged forced labor of a mentally disabled woman held captive 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 first-hand knowledge of the investigation.
They were also frequently denied access to the bathroom, FBI Special Agent Eric Smith told reporters on Tuesday.
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, 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.
"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.
Andy Hyde, who has represented Callahan, told CNN on Wednesday that prosecutors are wrongly characterizing the relationships among the alleged victims and the suspects.
"It was clear in my discussions that they were never forced. This group were all friends," said Hyde. "There are photographs ... (of) them all drinking beer together, laughing, joking on a couch. Traveling to other places together. They moved to several different residences together."
Investigators stated Callahan and Hunt in 2011 persuaded the mother to move into the apartment they shared with Hunt's four sons and "numerous" animals, knowing she "suffered from a cognitive disability and received monthly public assistance payments."
According to prosecutors, Callahan showed police a mobile-phone video of the mother, identified in court papers as S.E., beating her child.
S.E. told police that she had been told to do so by Callahan and Hunt and that Callahan threatened to show police the video if she "messed up" or went to authorities.
Hyde told CNN's Erin Burnett the mother has given different accounts, from denying she beat the child, to admitting to doing so once to saying she was forced to do it.
"Her story has changed so many times I think it strains credibility to believe what she is saying," Hyde said. "This is not a slave labor case at all. They were friends."
CNN's Carol Cratty, Matt Smith, Chandler Friedman and Catherine E. Shoichet contributed to this report.