California reels from two gun massacres

A deadly rampage by a 67-year-old suspect in northern California was likely an act of workplace violence, authorities say, offering fresh details about the second in back-to-back mass shootings that killed 18 people and horrified the nation.

In two separate shootings within three days, 11 people were killed at a Los Angeles-area dance hall and seven others were murdered at two farms in the seaside town of Half Moon Bay near San Francisco.

Authorities said they still were trying to determine motives for both shootings.

Preliminary evidence indicated the massacre in Half Moon Bay was a case of workplace violence and the gunman had used a legally-owned weapon. In the southern California shootings, officials have declined to make any statement regarding a possible motive.

The suspects in both attacks are men of retirement age, much older than typical perpetrators of deadly mass shootings that have become routine in the United States.

On Monday, a gunman in Half Moon Bay opened fire on employees at Mountain Mushroom Farm, where he had previously worked, and Concord Farms, about 1.6 kilometres away.

The suspect, 67-year-old Chunli Zhao, was taken into custody later after he was found sitting in his vehicle, parked outside a sheriff's station. Authorities said they believe he had come to surrender.

He killed seven and wounded one, and all his victims were Hispanic and Asian Americans, San Mateo County Sheriff Christina Corpus told reporters on Tuesday.

"All of the evidence we have points to this being the instance of workplace violence," Corpus said.

Zhao had threatened a co-worker in the past, according to court records obtained by the San Francisco Chronicle. In 2013, a roommate and colleague at a San Jose restaurant obtained a restraining order after alleging Zhao had assaulted him and made death threats. The court order is no longer in effect.

Zhao faces charges including deliberate and premeditated murder, attempted murder in the first degree, and discharging a firearm with enhancement for "most violent felonies".

Zhao was expected to be formally charged and appear in court on Wednesday.

President Joe Biden renewed his call for Congress to ban assault weapons.

"Even as we await further details on these shootings, we know the scourge of gun violence across America requires stronger action," he said in a statement.

News of the Half Moon Bay shooting broke as detectives were investigating the Saturday night shooting at Star Ballroom Dance Studio in Monterey Park, just east of downtown Los Angeles, which occurred in the midst of the predominantly Asian American city's Lunar New Year celebrations.

Huu Can Tran, 72, fatally shot 10 people and wounded 10 others at the Star Ballroom, where he was a frequent patron. An 11th person died in the hospital from their wounds.

Tran then drove to a second dance hall nearby, but was unable to inflict further damage because the club's operator wrestled his weapon away. He fled in a white cargo van.

Police cornered him about 12 hours later in Torrance, south of Los Angeles, and then he fatally shot himself.

Among the 11 victims were two fishermen from Taiwan, according to the Taiwanese consulate in Los Angeles, and multiple Chinese citizens, according to the Chinese consulate.

It ranked as the deadliest mass shooting ever in Los Angeles County. By comparison, the 1984 massacre of 21 people at a McDonald's restaurant in San Diego was the greatest loss of life from a single California shooting.

Monterey Park Police Chief Scott Wiese said investigators were looking into unconfirmed reports that the violence might have been sparked by jealousy or relationship issues.