A gunman killed 10 people at a ballroom dance hall during a Lunar New Year celebration near Los Angeles before fleeing the scene and later killing himself when approached by police on Sunday.
An 11th victim died on Monday in hospital as a result of gunshot wounds, according to Los Angeles County Sheriff Robert Luna.
Sheriff Luna said the motive for the shooting was not known. He identified the suspected gunman as Huu Can Tran, 72, who wielded a pistol with a high-capacity magazine.
“We want to know, we want to know how something this awful can happen,” the sheriff said.
Sheriff Luna said Tran turned the handgun on himself on Sunday morning as police approached a white van he was driving in Torrance, about 34km from the site of the shooting at the Star Ballroom Dance Studio in Monterey Park.
The shooting unfolded on Saturday night as tens of thousands of people had been attending Lunar New Year festivities in Monterey Park, which is 13km east of downtown Los Angeles.
“When officers arrived on scene, they observed numerous individuals, patrons … pouring out of the location, screaming,” said Capt Andrew Meyer of the Los Angeles County Sheriff's Department. “The officers made entry to the location and located additional victims.”
Capt Meyer said investigators had yet to determine if the victims were targeted and that it was too early to say whether the shooting was a hate crime.
“We will look at every angle,” he said.
Seung Won Choi told the Los Angeles Times that three people ran into his seafood restaurant and told him to lock the door as there was a man armed with a machinegun in the vicinity.
US President Joe Biden is monitoring the situation and receiving updates. He sent his condolences on Twitter.
Monterey Park has a population of about 60,000 and is home to a large number of Asian Americans.
Juno Blees took to Twitter to post a photo of her injured father lying in a hospital bed after the shooting. She said her mother was missing.
The two-day annual festival is one of the largest in southern California.
Police responded in huge numbers and helicopters were sent to the scene.
Footage showed injured revellers on stretchers being taken into ambulances.
Many reportedly mistook the shots for fireworks before realising an attack was taking place.
“Our hearts go out to those who lost loved ones tonight in our neighbouring city, Monterey Park, where a mass shooting just occurred,” Los Angeles city controller Kenneth Mejia said in a tweet.
Anti-Asian sentiment and violence in the US grew during the coronavirus pandemic, which originated in Wuhan, China.
Mr Biden's predecessor, Donald Trump, often called the illness the “China virus” or the “kung flu”. These remarks have been seen as stirring up anger against Asians among conservative Americans.
One in six Asian-American adults say they experienced a hate crime last year, up from one in eight in 2020, a survey by AAPI Data and Momentive showed.
Reuters contributed to this report