The attack took place in the city of Azusa, 40 kilometers east of Los Angeles, as Americans marked ballots in the presidential election.
Police officers who answered reports of the shooting were fired upon. The Los Angeles Times newspaper reports the officers shot at the suspect, who then entered a home. It is believed that no police officers were injured.
Azusa Police Chief Steve Hunt said officers were searching for at least one female suspect, and possibly another male suspect. He said he believed the woman was armed with a powerful rifle.
Hunt said a fourth person could be seen lying at the front door of a home. He said he could not say whether that person was the shooter or another victim.
Officials asked people to avoid the area and stay in their homes or offices. Two voting centers and a middle school were closed and guarded. People were not permitted to enter or leave them.