At least three people are dead and 27 others have been rushed to hospital after a boat was ripped into pieces when it capsized.
The 12-metre vessel was suspected of smuggling people when it broke apart off Cabrillo National Monument at California's Point Loma, San Diego Fire-Rescue said.
Lieutenant Rick Romero told reporters the boat got into trouble about 10am (local time) on Sunday before rescue crews received calls about 3pm.
After arriving on scene, emergency crews realised the boat had been ripped apart and dozens had been on board.
“Conditions were pretty rough: 5 to 6 feet (1.5 to 1.8 metres) of surf, windy, cold," Lt Romero said.
"There are people in the water, drowning, getting sucked out the rip current, and people on shore receiving CPR.
"There were about seven people in the water. Two of them were face down and drowned."
It was not immediately clear how the third person died.
Destroyed boat was a 'people smuggling vessel'
Video posted to Twitter shows rescuers in the water as broken debris is tossed by the waves.
Border Patrol agent Jeff Stephenson said the captain of the boat was in custody and speaking with investigators.
"Every indication from our perspective is that this was a smuggling vessel, used to smuggle migrants into the United States illegally," he said.
The nationality of the people on the boat was not immediately known,
Authorities said they had seen a marked increase in maritime smuggling in recent years.
The vessel involved in Sunday's incident was larger and had more people than most smuggling boats, according to the Border Patrol.
Cabrillo National Monument is a park with trails, tidal pools and exhibits on California history, operated by the US National Park Service.
It is named after Juan Rodriguez Cabrillo, who stepped ashore in 1542 as the first European to arrive on what is now the West Coast of the United States, according to the Park Service's website.
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