One person was killed and at least three others were injured in gunfire on Tuesday near a polling station in the southern California town of Azusa.
Authorities immediately lock down the polling place, a surrounding park and adjacent schools.
Los Angeles County Sheriff's Captain Jeff Scroggin said police were dealing with at least one suspect who was heavily armed.
The incident occurred outside the Memorial Park North Recreation Centre in Azusa which is located roughly 30 miles outside Los Angeles.
One of the injured was headed to the polling station to vote, a law enforcement official said.
The police arrived to find a man with a rifle. The gunman immediately fired at least 20 shots, a source said.
Under a hail of gunfire, police officials took cover and returned shots at the man, who retreated into a home in the 500 block of Fourth Street, it added.
Officers have the suspect surrounded and were evacuating the area.
Roberto Chavez, 67, and his wife were sitting outside on their porch when they saw a heavily-armed man fire at least 10 rounds at group of people.
The gunman, who wore a white shirt and black pants, ran into a home about three doors down from Chavez's house, he said.
Los Angeles County Registrar Dean Logan said the shooting was impacting two polling stations at Memorial Park, a preschool, and Dalton Elementary school.
He urged voters to avoid the area and "if necessary, cast a ballot at an alternate polling location".