The girl, who was entering high school in the fall, was hosting a sleepover Saturday night, Sonoma County sheriff's Lt. Dennis O'Leary said.
"We believe they were drinking alcohol," O'Leary said. "Right now we are investigating the death as suspicious, but we think it may be accidental."
O'Leary said results from an autopsy being performed Monday and toxicology tests could take up to two weeks.
Rao's mother told authorities she woke up early Sunday and saw two of the girls throwing up. They told her they were suffering from food poisoning as she helped clean them up and put them back to bed.
The mother woke up about an hour later when her daughter and another girl began vomiting. Hours later, two of the girls had already been picked up by their parents and a third was in the shower when Rao's parents tried waking their daughter.
Rao's friends later told investigators the girl had passed around a soda that had tasted strange, but they said they didn't know what was in it.
Authorities advised the girls' parents to take them to a hospital as a precaution.
"Obviously, parents need to talk with their kids and discuss the dangers of consuming alcohol and maintain an open line of communication," O'Leary said.