US crash investigators have revealed that the navy has been asked to search for a section of the MD-83 airliner about 4 miles (6 km) from the main wreckage site.

All 88 passengers on the flight from Puerto Vallarta, Mexico, to San Francisco and Seattle on 31 January were killed when the plane nosedived off the California coast.

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

The possibility that part of the plane was lost before the crash was disclosed by the chairman of the National Transportation Safety Board, Jim Hall at a news conference.

"Primary radar hits might be indicative, and I emphasise might be indicative, of something coming off Flight 261," he said.

A loud noise was also heard on the cockpit voice recorder at about the same time as the start of the final descent, he said.

Stabiliser recovered

Mr Hall added that the navy had recovered an 8ft section of what the NTSB believed had been the left portion of the plane's horizontal stabiliser and a portion of the centre section on Monday.

For at least half an hour before the crash, the crew reported problems controlling the plane's horizontal stabiliser - the wing-like structure mounted at the top of the tail in the MD-80 series of aircraft.

Flight 261's stabiliser went to a full nose down position slightly more than 12 minutes before the plane crashed.

The elevator panels on the trailing edge of the stabiliser can be used to overcome a runaway stabiliser and the safety board said the crew successfully did that for a while.

Even if something did break off the plane, safety board officials will still need to determine if the event caused the final dive or was just a result of the plane's extreme motion.

Fatal plunge

Mr Hall said the plane plunged 18,000ft (6,000m) in little over a minute.

Meanwhile, three planes of similar design have been forced to land after experiencing in-flight problems in the last eight days.

BBC correspondents say it is quite possible that crews may have been testing their plane's stabilisers, causing them to overheat.

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