By Sunday afternoon, the shooter had not been captured.
The Los Angeles County Sheriff's department released surveillance camera images, showing the unidentified suspect wearing glasses, in a dark jacket and a winter hat, and said he should be considered "armed and dangerous."
"We need to get this person off the street as soon as possible."
Sheriff Robert Luna on Sunday said officers rushed to the ballroom around 10:20 Saturday night, and found victims pouring into the parking lot.
"Additional officers made entry into the business and located numerous gunshot victims. // And I do want to point out these police officers that get there, they don't hesitate - they go in right away."
Five women and five men were killed, Luna said, and at least 10 people were taken to local hospitals. Police have not said what kind of gun was used in the attack.
The celebration in Monterey Park - home to one of the largest Asian American communities in the United States - was part of a weekend of Lunar New Year festivities that drew thousands to the city.
Luna said Sunday it was too early to determine a motive for the attack.
"We don't know if this is specifically a hate crime defined by law, but who walks into a dance hall and guns down 20 people? The description we have now is of a male Asian. Does that matter? I don't know. I can tell you that everything's on the table."
Officials were investigating whether an incident at another dance club in the neighboring city of Alhambra Saturday night was connected.
Witnesses there said an Asian man walked in holding a gun, about twenty minutes after the first shooting. Patrons were able to grab the weapon, Luna said. No one was shot and the man fled.
The White House said President Joe Biden had been briefed on the attack and had directed the FBI to assist local police.
