The shooting occurred around 6pm inside an ice cream shop in a strip mall where the child's birthday was being celebrated, according to the San Joaquin County Sheriff's Office.
An unidentified individual suddenly opened fire during the celebration, authorities said. Officials confirmed that 14 people were shot in the incident, while four victims died at the scene. The victims reportedly include children as well as adults.
The crime scene has been sealed off by law enforcement and investigative teams are collecting evidence from the site.Read More: National Guard shooting suspect served alongside US army in Afghanistan: US media
Police said that so far, no information about any suspect has emerged in the case. However, they added that investigations are being carried out on a priority basis as authorities work to determine the circumstances of the shooting.
Deputies said that "approximately 14 individuals were struck by gunfire, and four victims have been confirmed deceased."
San Joaquin County District Attorney Ron Freitas and Mayor of Stockton Christina Fugazi have both indicated that children are among the victims who died. That information has not yet been confirmed by the sheriff's office.
Heather Brent, a spokesperson for the San Joaquin County sheriff's office, said the gunshot victims included both children and adults.
Early indications "suggest this may have been a targeted incident," Brent said during a news conference at the scene.Detectives were still working to identify a possible motive.
Authorities did not immediately provide additional information about the conditions of the victims. Officials said earlier that several were taken to hospitals.