A woman died Thursday after a U.S. Customs and Border Protection (CBP) boat collided with a suspected people-smuggling vessel off the southern California coast.

Authorities said they spotted the small panga motorboat before dawn off the coast of Encinitas, Calif. They said it was crowded with 20 people believed to be trying to enter the U.S. illegally. Immigrant smugglers have been known to use open boats to sneak people from Mexico to the California coast.

CBP spokeswoman Jackie Wasiluk says agents ordered the skipper of the 26-foot-long boat to stop and fired warning shots when it kept going. The two vessels then collided, capsizing the panga and sending its occupants into the water.

Agents pulled the group out of the water and performed CPR on the unconscious woman. She was pronounced dead at a local hospital. The cause of her death was not immediately clear. Three other people were also hospitalized.

Wasiluk said it was not clear whether the pilots of the panga were transporting any other contraband. It was not clear whether the CBP boat had been seriously damaged.

Late Thursday, the Mexican government confirmed that the deceased woman was a Mexican citizen. A statement from the Foreign Relations Department said one other person was seriously injured and two others suffered less severe injuries.

An investigation into the incident is ongoing.

The Associated Press contributed to this report.

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