"Los Angeles County Sheriff's Homicide detectives are responding to assist Monterey Park Police Department with a shooting death investigation. There are nine deceased," the department said in a brief statement.
"There is no additional information available at this time."
The department said the suspect was male, but it was not clear if he was still at large.
The shooting took place after 10 p.m. (0600 GMT on Sunday) around the location of a Chinese Lunar New Year celebration held in Monterey Park, U.S. media reported.
Footage posted on social media showed injured people on stretchers being taken to ambulances by emergency staff. Around the scene of the shooting - reported to have been at a dance club - police guarded cordoned-off streets, the video showed.
"Our hearts go out to those who lost loved ones tonight in our neighboring city, Monterey Park, where a mass shooting just occurred," Los Angeles City Controller Kenneth Mejia said in a tweet.
Tens of thousands of people had attended the festival earlier in the day.
The Los Angeles Times quoted the owner of a nearby restaurant as saying that people who sought shelter in his property told him there was a man with a machine gun in the area.
Seung Won Choi told the newspaper that he believed the shooting took place at a dance club.
Monterey Park is a city in Los Angeles county, around 7 miles (11 km) from downtown Los Angeles.
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