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Underwater zone expands in search for missing Malaysian plane

TOKYO — The sputtering search for a missing Malaysian airliner will be expanded to include a much larger swath of the Indian Ocean floor, Australia’s prime minister said Monday, signaling a daunting new phase in the bid to find the aircraft’s wreckage.

The next stage in the hunt for Malaysia Airlines Flight MH370 will focus entirely on underwater exploration of the depths, forgoing the use of airplanes and vessels to spot debris on the surface. By now, Prime Minister Tony Abbott said, any wreckage is likely to have become ‘‘waterlogged and sunk.’’

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A broader underwater search — requiring robotic miniature submarines trawling the depths at walking speed — casts doubt about the chances of ever unraveling a confounding aviation mystery.

It also drastically increases the cost and expands the timetable. The underwater search was previously focused on a small area that required two weeks to examine. The expanded area is about 22,000 square miles, about the size of West Virginia, and could require six to eight months to fully scour.

More than seven weeks into the search, the countries involved, including the United States, have been bearing their own costs. But Abbott said Monday that Australia will seek contributions from other nations while also engaging private companies, selected with help from the Malaysian government.

‘‘We will do everything we humanly can, everything we reasonably can, to solve this mystery,’’ Abbott said at a news conference in Canberra, the Australian capital.

No debris has yet been found from the airplane, which disappeared March 8 with 239 passengers and crew on board en route from Kuala Lumpur to Beijing. Its likely endpoint — the Indian Ocean off the west coast of Australia — was determined only from analysis of signals the plane transmitted to a satellite while still airborne.

Hopes for finding the plane picked up earlier this month, when US Navy equipment picked up a series of deep-sea pings that appeared to be coming from the airplane’s black box. The flight data recorder and cockpit voice recorder are equipped with batteries to power emergency beacons after a crash.

But the search hit a wall. Black box batteries have a guaranteed shelf life of just 30 days, and the US Navy equipment stopped picking up the signals.

Then the Bluefin-21, a torpedo-like robotic submarine built in Quincy, Mass., was sent to the 2.8-mile depths, but it found nothing of interest.

Abbott said officials will bring in equipment in addition to the Bluefin-21 for the next phase of the search. But the process of organizing contracts with private companies could take several weeks, and in the meantime the Bluefin-21 will still scour the ocean floor.

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