Farm worker charged over US gun rampage

A 66-year-old farm worker has been formally charged with premeditated murder in the fatal shooting of seven co-workers near San Francisco, the second of two gun rampages in California in recent days in which 18 people were killed.

Chunli Zhao, a Chinese citizen and the lone suspect in Monday's massacre at two mushroom farms in the seaside town of Half Moon Bay, was formally presented with seven counts of murder and a single count of attempted murder during his first court appearance in nearby Redwood City.

Zhao, wearing red jail garb and enclosed behind a glass panel, was ordered held without bond during a brief hearing in San Mateo County Superior Court on Wednesday.

No plea was entered and the case was adjounred to February 16.

A Mandarin-language translator was provided for the defendant, who according to District Attorney Steve Wagstaffe is a Chinese citizen who has lived in the United States for at least 10 years.

After the hearing, Wagstaffe told reporters outside the courthouse that prosecutors had not yet determined Zhao's precise immigration status, or whether he entered the country legally.

The prosecutor said authorities had an idea about the suspect's motives but declined to share any details.

The criminal complaint alleges "special circumstances", which means if convicted of murder Zhao could be eligible for the death penalty. However, Governor Gavin Newsom in 2019 declared a moratorium on executions.

Otherwise, the maximum sentence is life in prison without the possibility of parole.

Late on Wednesday, Vice-President Kamala Harris paid a condolence visit to the Los Angeles suburb of Monterey Park, where 11 people were killed in a separate rampage at a ballroom dance hall on Saturday night by gunman Huu Can Tran, 72, who later took his own life.

Harris, a California native, laid flowers at a memorial set up outside the dance studio, and in brief comments to reporters called for Congress to enact tougher national gun-safety measures. She then met privately with victims' families.

Coming in quick succession, the two shootings left California reeling from one of the bloodiest spates of mass gun violence in decades in a state with some of the strictest firearm laws in the country.

Authorities said that each of the two killing sprees represented the single greatest loss of life from a single act of violence in Los Angeles and San Mateo counties, respectively.

Asked if investigators believed the Half Moon Bay killings were a copycat crime, inspired by the shooting rampage in Monterey Park two days earlier, Wagstaffe said flatly, "No."

Zhao was arrested on Monday evening outside a sheriff's station, where police said he had driven shortly after the attack.

He had been employed by one of the growers, Mountain Mushroom Farm, and had resided at the property along with other employees, according to California Terra Gardens, which owns the farm.

Authorities said early evidence indicated the bloodshed stemmed from a workplace grievance. The second crime scene, Concord Farms, is about 1.6 kilometres away.