Crews of boats and helicopters are searching for a Navy helicopter with five people aboard that crashed into the ocean some 13 miles (21 kilometers) south of San Diego.
There was no word early Wednesday on whether any wreckage or survivors had been found.
Navy Lt. Candice Tresch in Washington said the SH60 Seahawk helicopter went down during a training exercise Tuesday night.
The crash area is off the northwest Mexican coast, near the Coronado Islands.
The Coast Guard said the search for survivors involved helicopters and boats from the Coast Guard, Navy, US Customs and Border Protection, and the San Diego Harbor police.