Los Angeles County Sheriff's detectives said they are treating the case as a homicide. They are awaiting autopsy results to determine how she died.
Authorities said the toddler lived at the home with her mother and her biological father. Neighbors said the mother lived at the home for a couple of years but the girl's father had recently moved in.
Neighbor Joel Ramirez said he had never seen any signs of trouble before Friday.
"I saw the paramedics take the little girl to the hospital," Ramirez said. "When I was looking, the little girl looked in bad, bad shape. We don't really know what happened. Everybody was feeling real sad."
No arrests have been made.
Detectives said they were interviewing everyone in the home to figure out what happened to the toddler.