Authorities said they had not identified the motive for either of the rampages, which seemed especially baffling in part because the suspects in both were men of retirement age, much older than is typical for perpetrators of deadly mass shootings that have become numbingly routine in the United States.

California Governor Gavin Newsom said he was visiting wounded survivors from Saturday night’s massacre in the Los Angeles suburb of Monterey Park when he was informed of Monday’s killings in northern California.

“Tragedy upon tragedy,” he wrote on Twitter.

Otherwise, the back-to-back shootings appeared to have little in common.

The latest spasm of gun carnage struck the coastal town of Half Moon Bay, near San Francisco, where a gunman opened fire on groups of farm workers at two locations, leaving a total of seven dead and one badly wounded, then fled.

The accused gunman, identified as Chunli Zhao, 67, was arrested a short time later after he was found sitting in his vehicle, parked outside a sheriff’s station, where authorities said they believe he had come to turn himself in.

A semi-automatic handgun was found in his car, San Mateo County Sheriff Christina Corpus told an evening news conference.

Corpus said the suspect worked at one of the two crime scenes, which she described as agricultural “nurseries”.

Local media described one site as a mushroom farm.

In a separate Bay-area incident on Monday evening that drew far less attention, one person was killed and seven wounded in a “shooting between several individuals” in Oakland, police reported, in circumstances suggesting a case of gang violence.

News of the massacre in Half Moon Bay surfaced as police worked through a second full day of their investigation into the shooting at the Star Ballroom Dance Studio in Monterey Park, just east of downtown Los Angeles, where a gunman shot and killed 11 people.

Nine others were wounded.

Authorities said the suspect, Huu Can Tran, 72, drove next to an adjacent town and barged into a second dance hall but was confronted by the club’s operator, who wrestled the weapon away during a brief scuffle.

Tran, himself a longtime patron of the Star Ballroom, catering mainly to older dance enthusiasts, fled again and vanished overnight.

He shot himself dead in his parked getaway vehicle, a cargo van, on Sunday morning about 12 hours after his rampage as police surrounded him in the town of Torrance, south of Los Angeles, authorities said.

Saturday’s violence unfolded in the midst of a Chinese Lunar New Year celebration in Monterey Park, a hub of the Asian-American community in Southern California, giving rise initially to concerns the attack may have been racially motivated.

The second day of the event was cancelled.

Attesting to the firepower unleashed at the Monterey Park ballroom, investigators collected 42 bullet casings and a large-capacity ammunition magazine from the scene, Los Angeles County Sheriff Robert Luna told reporters on Monday.

He said a search of the suspect’s mobile home in a gated senior-living community in the town of Hemet, east of Los Angeles, turned up a rifle, electronic devices and items “that lead us to believe the suspect was manufacturing homemade” weapons silencers.

Police also seized hundreds of rounds of ammunition from the dwelling and a handgun from the suspect’s vehicle.

Adam Hood, who rented a home from Tran in the Los Angeles area, told Reuters his landlord enjoyed ballroom dancing and was a regular at the Star Ballroom, although he complained others there were talking behind his back.

“He was distrustful of the people at the studio, angry and distrustful. I think he just had enough,” Hood said.

-AAP