A civilian vessel reported seeing the plane hit the water on Friday afternoon about 25 miles off the coast, halfway between Oceanside and San Clemente Island. Searchers found aluminum debris and an oil slick, Coast Guard Lt. Josh Nelson said.
Three rescue boats will search for survivors through early Saturday morning and a helicopter will be sent out after sunrise.
Federal Aviation Administration officials believe the plane took off from San Diego's Montgomery Field
- a popular site for small aircraft - but were still trying to confirm, Nelson said. He said there were no reports of overdue or missing commercial or military aircraft.
It was not immediately clear how many people were aboard the downed plane.
Oceanside is about 35 miles north of San Diego.