Two small planes have collided in mid-air and plunged into the ocean off Los Angeles harbour.
There was no immediate word of survivors, authorities said.
The planes apparently went into the water about three kilometres outside the harbour entrance, US Coast Guard and other officials said. It was unknown how many people were on board.
Fire and lifeguard boats and rescue divers swarmed the area.
"They found both a partial tail number and then a second tail number," Los Angeles County lifeguard Capt. Ken Haskett said.
As darkness fell about two dozen divers were on call to scour the area but the planes may be at the bottom in up to 30 metres of water, Haskett said.
The crash site was about 0.5 kilometres south of the Angels Gate light. The harbour entrance was closed to traffic while the search continued.
