The witnesses said the strike hit with a tremendous boom about 2:30 p.m. Sunday, rattling buildings and showering a lifeguard headquarters with sparks.
"The first knowledge they had was when the lightning hit," Capt. Danny Douglas of the Venice lifeguard station said Monday.
The 21-year-old was rushed unresponsive to a hospital after the strike and later died. Coroner's Assistant Chief Ed Winter identified him as Nick Fagnano of Los Angeles. Some witnesses said Fagnano had been in the water when the lightning hit, but authorities couldn't confirm that.
The lightning came as some 30,000 people were enjoying a day at the city's funky bohemian beach noted for its jugglers, skaters, medical marijuana dealers and boardwalk preachers and hucksters.
Swimmers cooling off on a muggy day, volleyball players on the sand and people strolling the famous boardwalk were jolted.
Twelve other people, including a 15-year-old boy, were examined after they felt the effects of the lightning, ranging from anxiety to a man who needed CPR. However, not all were necessarily actually struck by lightning, said Katherine Main, a city fire spokeswoman.
Nine were taken to hospitals, where one was listed in critical condition. Most of the others were mainly shaken up and expected to recover, fire officials said.
Lightning also struck a 57-year-old man on a golf course on Santa Catalina Island, the picturesque Channel Island 26 miles off the coast of Los Angeles that has been celebrated in story and song. He was listed in stable condition and further information on him was not available Monday.
Stuart Acher said he was shocked while playing volleyball on the beach.
"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."
Paramedics examined Acher but he felt all right and went back to playing volleyball.
Steve Christensen said his friend had been body-surfing and was sitting on the beach when lifeguards began searching for a missing swimmer.
"He (Christensen's friend) went out to the water to find him and walked right into him," Christensen said. "He was face down on the bottom."
Christensen said his friend pulled the man, who appeared to be in his 20s, from the water, and lifeguards began CPR before taking him away.
"This tragedy reminds us that we can take nothing for granted or underestimate the power of nature," Mayor Eric Garcetti said in a statement.
Hundreds of lightning strikes were reported around Southern California as a moisture-laden monsoonal flow spread up from the south and swept the region all the way out to the ocean.
"This is pretty rare" because usually the flow affects just the deserts and sometimes the mountains, said Stuart Seto, a weather specialist with the National Weather Service in Oxnard.
The storms began to dissipate as they moved northwest, leaving just a chance of storms through Monday, mainly in the deserts and mountains, Seto said.
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