Seven people have been killed in shootings in California’s second mass shooting in less than two days.

Four people died at a mushroom farm and three more at a trucking business about three kilometres away on the outskirts of Half Moon Bay, a city about 50 kilometres south of San Francisco, on Monday (US time), officials said.

An eighth person is in hospital in a critical condition.

Officials said the latest deadly attacks were related, although they are yet to confirm the connection. They came as the toll from Sunday’s deadly shootings at Lunar New Year celebrations at two dance clubs in Los Angeles rose to 11.

San Mateo County Board of Supervisors president Dave Pine said police had arrested 67-year-old Chung Lee Zhao in connection with the Half Moon Bay shootings.

It was not immediately clear how the locations were connected, though Mr Pine said the suspect worked for one of the businesses.

He called the suspect a “disgruntled worker”.

Police said Zhao was in custody and co-operating. But they did not yet have a motive for the rampage.

San Mateo County Sheriff Christina Corpus said witnesses to the attacks included children who who lived on the rural property.

“For children to witness this is unspeakable,” she said.

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California senator Josh Becker, who represents the area, said people were killed in separate shootings.

The San Mateo County Sheriff’s Office tweeted just before 5pm (local time) that a suspect was in custody.

“There is no ongoing threat to the community at this time,” the sheriff’s office said.

Television footage from the area showed officers taking a man into custody.

“Zhao was located in the parking lot by a sheriff’s deputy,” San Mateo County Sheriff Christina Corpus said.

“He was taken into custody without incident and a semiautomatic hand gun was located in his vehicle. He is believed to have acted alone and there’s no further threat to this community.”

Aerial vision also showed police officers collecting evidence from a farm with dozens of greenhouses.

The shooting followed the killing of 11 people late on Saturday at a ballroom dance hall in southern California.

The 11th victim died on Monday from injuries suffered in the weekend shooting rampage, as investigators search for a motive behind one of California’s bloodiest outbursts of mass gun violence.

Police identified Huu Can Tran, 72 as the lone suspect in a massacre that unfolded on Saturday night in Monterey Park, just east of Los Angeles.

Tran drove to another dance hall where a second attack was thwarted when two people wrestled his weapon from him. He later took his own life as police closed in to make an arrest hours after the shooting.

The Half Moon Bay attacks were the US’s sixth mass shooting, just 23 days into the new year.

Half Moon Bay is a small coastal city with agricultural roots that is home to about 12,000 people. The city and surrounding San Mateo County area is known for producing flowers and vegetables. The county allows cannabis farming in certain areas.

“We are sickened by today’s tragedy in Half Moon Bay,” Mr Pine said.

“We have not even had time to grieve for those lost in the terrible shooting in Monterey Park. Gun violence must stop.”

California Governor Gavin Newsom tweeted that he was “at the hospital meeting with victims of a mass shooting when I get pulled away to be briefed about another shooting. This time in Half Moon Bay. Tragedy upon tragedy”.

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-with AAP