Amanda Brumfield -- Thornton's 29-year-old daughter by his first wife Melissa Gatlin -- was arrested on Friday.
Brumfield told investigators the 1-year-old child she was looking after last October had fallen from a playpen and hit her head, but she did not call for help for over two hours, according to police reports.
Investigations found discrepancies in the babysitter's story, and an autopsy determined the child had a fractured skull and a subdural hematoma due to blunt trauma.
The death was ruled a homicide because authorities failed to believe elements of Brumfield's story.
A spokesman for Ocoee Police Department in Florida tells the Orlando Sentinel newspaper, "It might have been an accident, but she (Brumfield) had an obligation to get that child care. For two and a half hours she did not care for that child (by not calling for medical help)."
A statement from Thornton, who is estranged from his daughter, reads, "Anytime a baby's life is lost is an unimaginable tragedy and my heart goes out to the baby's family and loved ones."
(c) 2009 WORLD ENTERTAINMENT NEWS NETWORK LTD. All global rights reserved. No unauthorized copying or re-distributing permitted.