Both children and adults are among the victims, according to Heather Brent, a spokeswoman for the local sheriff's office.
"This may have been a targeted incident," she said at a press conference in Stockton.
The shooting took place during a family gathering in a party venue on Saturday evening, local time. Police are investigating and working to gather information and clarify the motive.
No one has been arrested after the shooting.
"Our biggest priority right now is identifying the suspect," said spokeswoman Heather Brent, NBC News reports.
Stockton has around 325,000 residents and is located east of San Francisco.
