Lightning struck on crowded Venice Beach in Los Angeles, California, killing one man and injuring several people.
The lightning struck amid a rare summer thunderstorm, spreading panic among bathers and visitors.
Lifeguards fanned out across the beach and the water to attend to the injured, many of whom were treated at the scene.
Elsewhere in California, emergency crews are battling to contain wildfires that are threatening hundreds of homes.
At least 14 homes have reportedly been destroyed in the fires, which have blazed across drought-stricken grassland and forest.
A fire in the Sacramento region has spread to cover an area of about 4,000 acres, while another blaze has been threatening homes around Yosemite National Park.
The man killed at Venice Beach is said to have been 20 years old. The exact cause of his death is not yet known, and it is unclear if he was struck directly.
Witnesses say the sky darkened suddenly and screams filled the air as the storm hit the beach on Sunday afternoon. Eight people were admitted to hospital.
One of the injured is said to be in a critical condition. Several people received treatment for milder symptoms, including anxiety.
Stuart Acher told KABC-TV he was hit by lightning while playing volleyball.
“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 the station.
“It went down the 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.”
Lightning also hit Catalina Island, near Los Angeles.
A 57-year-old man who was playing golf was injured in the strike. His condition is said to be stable.
Until the latest strike, at least 15 people had been killed this year by lightning in the US, according to the National Weather Service.
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