There are 11 known victims, local officials said, and the gunman is deceased.
The suspected shooter was found dead near a bridge with a self-inflicted gunshot wound, authorities say.
Thousands attend funeral prayers for the three victims of Monday's deadly shooting at the Islamic Centre of San Diego.
The child fired a single shot at a teacher, who had to have surgery after the incident in 2023.
Authorities say the teenage suspects shared a "broad hatred" of multiple religions and racial groups.
Security guard Amin Abdullah, one of three killed in the attack, was described as "a shining light".
Two teenagers fatally shot three men at an Islamic centre on Monday before taking their own lives, police say.
Two teenage attackers fatally shot three men at a mosque in San Diego, California, in a suspected hate crime, before taking their own lives, say police.
The alleged attackers were aged 17 and 18 and one left a note containing "generalised hate rhetoric", investigators say.
