Four people were killed and more than two dozen were injured when a boat suspected of attempting to smuggle migrants into California overturned on Sunday off the coast of San Diego, authorities said.

The incident took place at around 10 a.m. local time (1700 GMT) off the coast of Point Loma Peninsula near the Cabrillo Monument, which is San Diego's only National Park, the San Diego Fire-Rescue Department said.

Lifeguards received the initial notification of the vessel in question experiencing trouble via VHF radio from a tow vessel that was in the area. Multiple agencies including U.S. Coast Guard immediately responded to the scene. 

The 40-foot (12-meter) cabin cruiser pummeled into pieces by the surf by the time rescue boats arrived, said Rick Romero, lifeguard lieutenant for San Diego Fire-Rescue. 

"When we arrived on the scene... there were people in the water, drowning, getting sucked out of the rip current," he said.

Several people were pulled from the water, some of which required life-saving assistance on the beach. 

There had been about 30 people on the boat in severely crowded conditions without adequate safety equipment, authorities said.

"Every indication from our perspective is that this was a smuggling vessel, used to smuggle migrants into the United States illegally," said Jeffery Stephenson of the San Diego sector of U.S. Customs and Border Protection.

The nationalities of the people on the boat were not immediately known, while the captain is in custody and speaking with investigators, Stephenson said.

Four days earlier, U.S. authorities detained 21 Mexican nationals after a panga boat was stopped off about 11 miles west of the Point Loma shoreline, reported KGTV, an ABC-affiliated television station.

"We've seen a dramatic increase in the number of maritime smuggling attempts recently," Aaron Heitke, chief patrol agent of the U.S. Border Patrol's San Diego Sector, was quoted as saying by the news outlet.

The U.S. Border Patrol has documented 157 maritime smuggling events in the San Diego area in this fiscal year, KGTV reported, citing officials. 

(With input from Reuters and Xinhua)