The shooting took place at a child's birthday party, Stockton's Vice Mayor Jason Lee said in a Facebook post.
"I am in contact with staff and public safety officials to understand exactly what happened, and I will be pushing for answers," he said.
Police said they received reports shortly before 6pm Saturday local time (0200 GMT) of a shooting near the 1900 block of Lucile Avenue in Stockton.
"We can confirm at this time that approximately 14 individuals were struck by gunfire, and four victims have been confirmed deceased," the San Joaquin County Sheriff's Office said in a post on X.
"This is a very active and ongoing investigation, and information remains limited. Early indications suggest this may be a targeted incident, and investigators are exploring all possibilities."
Sheiriff's office spokeswoman Heather Brent said the victims included children and adults.
Early indications "suggest this may have been a targeted incident", Brent told reporters at the scene.
The shooting occurred inside the banquet hall, which shares a parking lot with other businesses.
Detectives were still working to identify a possible motive.
Authorities did not immediately provide information about the conditions of the victims.
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