Officers with the Oakland (CA) Police Department received a piece of evidence on Friday that they initially thought was a Halloween hoax—a human skull.

People visiting an Oakland apartment complex last week found the head in the grassless yard with a couple of trees and took it to a police station, according to CBS News.

"I can say in my years of service, I've never had a human skull delivered to the police station," Sergeant Michael Cardoza told KGO-TV.

The Alameda County Coroner's Bureau is now working to determine the identity and cause of death.