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Southern California lifeguards rescued 13 people and were searching for others believed to have been dropped off early Friday by Mexican migrant smugglers in the Pacific Ocean, authorities said.

As many as three people remained missing based on information from the group of men and women rescued along the coast at Del Mar, an upscale beach town about 30 miles (48 kilometers) north of the Mexico border, U.S. Border Patrol spokesman agent Jarrett Decker said.

Over the past decade, smugglers have increasingly turned to using the sea to get people into the U.S. and avoid trying to avoid multiple barriers on land along the California-Mexico border and highway checkpoints. Smugglers can charge considerably more money taking people by sea.

A few migrants have been killed when boats overturned in the San Diego surf. Others have been found clinging to an overturned boat.

“This is extremely dangerous,” Decker said.

All 13 were taken to a hospital to be treated for hypothermia, Decker said. The group told authorities there were two smugglers on board who told them to swim ashore.

Coast Guard crews flying overhead were also attempting Friday to locate the boat known as a “panga” that the smugglers used to transport the migrants, though it is suspected it returned to Mexico, Decker said.

Such single-engine wooden skiffs are favored by smugglers who often try to get their passengers ashore as quickly as possible and return to Mexico to avoid being caught and losing their boats. The have been known to quickly abandon the boats if they sense they’re about to be caught.

Firefighters and lifeguards responded Friday after receiving reports about 4:30 a.m. of people in the ocean calling for help in Spanish. They were later assisted by the San Diego County Sheriff’s Department, U.S. Coast Guard, Border Patrol and other agencies.

Officials were still identifying the nationalities of those rescued. Of the 13 in custody, eight are men and five are women.

So far this year, U.S. authorities in San Diego have detained 368 people attempting to enter illegally into the U.S. via maritime smuggling, according to the U.S. Border Patrol.