One person was killed and at least three others injured as an armed gunman opened fire near a polling station in Azusa, in Los Angeles County, on Tuesday.
The incident has prompted the authorities to put two polling sites on a lockdown.
"Situation has transitioned from active to barricade/containment... All those near scene shelter in place all others stay out of the area," the Azusa police later tweeted.
The Los Angeles Times quouted eye-witnesses as saying that the assailant was a man.
"It's very early to tell what the motive is, but it appears to be concentrated on the residential side, not at the polling station," Azusa police chief Steve Hunt told the daily.
The police chief said that the suspect was armed with an assault rifle with "a rapid fire capability," of with 20 shots were fired at the police. However, no officers were injured during the shootout. Azusa Police Officer Jerry Willison has called it "a very volatile and critical situation."