Fairfield crime scene investigators spent all of Friday at Allen Witt Park where the body of a teenage girl was found around sunrise.
Sgt. Rob Lenke of the Fairfield Police Department says a police officer was doing a security check of the park at about 6:45 a.m. when a person flagged him down and told him about the body.
Police say the girl was unclothed but did not show any apparent signs of trauma. On Saturday they said they know how she died, but would not release the details.
Police told NBC Bay Area that the teenager was from Suisun City and had been reported missing Thursday by her guardian.
Her name was not being released, but NBC Bay Area learned she was a middle school student in the Fairfield Unified School District who was just 13 years old. She was living with a foster family in Suisun City prior to her death.
Police said the girl went to school Thursday, but never made it home and that police in Suisun City searched for her Thursday night.
"Obviously anytime you have a report of a dead young female everybody is very concerned as are we. Unfortunately right now we don't have any sort of identification," Lenke said.
An autopsy is being conducted as officers continue to scour the park for evidence.
People were leaving flowers and candles at the park this weekend to memorialize the girl.