Police were attending a shooting in Monterey Park, California with multiple casualities on Saturday night, the Los Angeles Times reported, citing a law enforcement source.
The shooting took place after 10 p.m. (0600 GMT) around the location of a Chinese Lunar New Year celebration held in Monterey Park, the newspaper said.
Tens of thousands of people had attended the festival earlier in the day.
Footage posted on social media showed deserted, cordoned-off streets guarded by police cars.
Monterey Park is a city in Los Angeles county, around 7 miles (11 km) from downtown Los Angeles.