SAN DIEGO -- An American and a German were arrested Sunday on suspicion of trying to smuggle 15 Mexican nationals into the South Bay on a 35-foot sailboat, federal officials said Tuesday.

The arrests came after a National Guardsman on duty in Imperial Beach notified Border Patrol agents about a vessel crossing into U.S. waters about 5:20 a.m. Sunday, according to U.S. Customs and Border Protection.

When law enforcement agents approached the boat about 1 1/2 miles off Border Field State Park, the guardsman reported people scrambling to go below deck, CBP officials said.

The agents boarded and seized the vessel, arresting a German and an American whose names were withheld, according to CBP officials.

It was unclear if any of the would-be immigrants were arrested.

It was the second time in little more than a week that members of the National Guard alerted Border Patrol agents to illegal immigrants.

On Sept. 25, a National Guard helicopter pilot notified agents that a northbound SUV was stuck in the border fence a few miles east of the Otay Mesa port of entry, CBP Agent Matthew Johnson said.

The pilot reported about 10 people fleeing the vehicle and running back toward Mexico, Johnson said, adding that agents later found a blow torch near the corrugated steel fence, which was cut open.