An intact section of the plane's tail and a piece of fuselage have been found. Several large pieces of debris were also located using underwater robots.

On the tail fin the Alaska Airlines logo - a smiling Eskimo - is clearly visible, said John Hammerschmidt, of the National Transportation Safety Board (NTSB) which investigates US air crashes.

Both black boxes - the flight data recorder and cockpit voice recorder - have been recovered and are being examined by investigators.

An initial review of the cockpit conversations has begun to reveal the pilots' desperate struggle to save Flight 261.

Click here for a graphic of the last minutes of flight 261

NTSB chairman Jim Hall said that during the final minutes the plane was flying upside down before going into a corkscrew and plunging into the ocean, killing all 88 people on board.

The plane had been heading for an emergency landing at Los Angeles when it crashed on Sunday.

Stabilser theory

Mr Hall said the cockpit conversations, in which the pilots reported problems with the stabilising systems on the plane, were consistent with eyewitness accounts of the crash.

The flight data recorder should show the positions of the plane's controls as the pilots wrestled to bring the aircraft under control.

Mr Hall said: "The crew had difficulty controlling the airplane's tendency to pitch nose down. The airplane descended, but the crew was able to arrest the descent.

"The crew continued troubleshooting and preparing the airplane for landing, then control was suddenly lost. The crew made references to being inverted that are consistent with the witness statements to that effect."

Dozens of boats have stayed in the area to help the search for wreckage and an ocean tug should arrive on Friday, providing investigators with a diving platform and towing options.

Memorial service

More than 100 friends of relatives of the victims held a memorial service on the beach facing the Santa Barbara Channel for a memorial service.

They lit candles and comforted each other, while others left bouquets of roses on the sand. Another memorial is planned for Saturday.

So far only a handful of bodies have been recovered. Investigators believe most are pinned beneath the debris of the plane.

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