A polling station in the California city of Azusa is in lockdown after shots were fired nearby.
The local police department reported at around 3pm local time (11pm GMT) that a 'critical incident' had taken place, and all officers had been deployed to the scene.
CBS quoted one officer as saying his colleagues had been met with gunfire and been "pinned down" by an unnamed attacker.
At least two people have been confirmed injured in the attack. The Los Angeles Times now reports one person has died while three others are injured, although this has yet to be confirmed.
It has been reported that several people, including a group of children, have been locked in a local voting station by security officials and forbidden from leaving while the shooter is active.
One voter told CNN that he was inside the polling booth when the shots were heard. He said a "massive shooting is happening and police have told us to stay indoors. We are currently on lockdown."
Another voter said: “At first I thought it was construction but people came running into the room saying they see a guy with a bullet proof vest and a white shirt. as of right now they just have us in the voting room and are trying to keep us calm.”
Those trapped inside have reportedly been allowed to continue voting while the lockdown is in place.