The shooting took place in early afternoon yesterday in the city of Azusa, 25 miles (40 kilometers) east of Los Angeles, Vanessa Lozano, spokeswoman for the Los Angeles County Fire Department told AFP.
She said three people were transported to a hospital with gunshot wounds and one was dead on arrival.
Details about the incident, which happened in a residential neighborhood, were sketchy.
A polling station in the area was placed on lockdown as was a school, according to local news reports.