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A 20-year-old man has died after lightning struck 14 people at Venice Beach in Los Angeles and Catalina Island.
Twelve other people were injured on the beach, with one still in a critical condition in hospital.
A 57-year-old man was also hit on a golf course on Catalina Island.
A witness earlier described seeing a "big flash of light and a boom" when the lightning struck.
Witnesses have described the aftermath of a lightning strike in California that left 14 people injured, two critically.
Stuart Archer, who was struck while playing volleyball on the beach, said it felt as if someone punched him in the back of the head.
"We went about our game and then all of a sudden, there was a big flash of light and a boom, and it felt like someone punched me in the back of my head," he told KABC-TV.
"It went down my whole side of my right body, and my calves sort of locked up, and I fell over. And I looked up and everybody else was, you know, falling over."
Jerome Williams said the lightning sounded like a "sonic boom", adding: "The majority of the folks that were on the (volleyball) court all felt a little tingly."
At least 14 people have been hit by lightning during thunderstorms in southern California, leaving two people in a critical condition in hospital.
Thirteen people - including a 15-year-old were struck at Venice Beach in Los Angeles on Sunday afternoon.
A 57-year-old man was injured by the strikes on a golf course on Catalina Island.
He is reported to be in a stable condition.
Eight people have been struck by lightning on Los Angeles' popular Venice Beach following a series of thunderstorms in the region.
A ninth person - reported to be a 57-year-old man - was taken to hospital after he was hit on a on a golf course on southern California's Catalina Island, authorities said.
Venice Beach experienced the extreme weather at around 2pm on Sunday, with a 15-year-old among the casualties.
The lightning strikes also sparked small brush fires on Catalina Island.