The name of the girl, a foster child from Suisun City who attended Green Valley Middle School had not been released, the San Francisco Chronicle reported.
Police officer Cleo Mayoral told the newspaper the cause of death had been determined by the Solano County coroner but was being withheld, along with other details.
"It is a very intense investigation and it's going non-stop," Mayoral said. "We have all of our resources working on this."
Mayoral said no suspects had been identified.
The girl had been reported missing Thursday night and her body was found by a citizen Friday morning in a parking lot in Allan Witt Park, prompting police to begin a Sacramento's KOVR-TV reported.
Sacramento's KXTV reported authorities said there were no obvious signs of trauma on the girl's body.