The search team spent the day walking the creek, looking for the rest of the skeleton. So far, they've found a lot of animal remains, but no human bones.
What they do know is that the skull is not Native American because of its dental work.
Investigators are working on the premise that the skull rolled down the creek. But they're not sure how far it traveled so they've been searching upstream one section at a time. However, there are several possibilities.
"It could have been buried there and the rest of the body washed down, or it could have been upstream and ended up here. So that's exactly what we're working through. As you can tell it's a large area to search," said Sgt. J.D. Nelson of the Alameda County Sheriff's Office.
A forensic anthropologist says the skull belongs to an adult and now investigators are working to determine the gender. They believe the skull has been in the creek area for at least a year so police are also busy combing their missing person reports.