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HONG KONG — The search for Malaysia Airlines Flight 370 suffered another setback Thursday when the communications system for a U.S. deep-sea submersible broke down and the Australian vessel that had been towing it in the Indian Ocean was forced to head back to port.

The Australian ship, the Ocean Shield, had set out from port last Saturday on its second voyage to find the missing Boeing 777. It reached the search area and lowered the submersible, a Bluefin-21, on Wednesday afternoon to resume the task of scanning the dark ocean floor with sonar for possible debris from the aircraft.

But soon after, the operators of the Bluefin, who were aboard the Ocean Shield, encountered difficulties in communicating with the submersible, according to a statement released Thursday by the Australian government’s Joint Agency Coordination Center.

The Bluefin was raised from the depths just two hours after it had been deployed, but was damaged in the process of being recovered from the water. Specialists aboard the Ocean Shield were able to repair the damage with spare parts on the ship.

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The communications problems, however, proved more intractable. An examination of the equipment showed that the communications transponder aboard the Ocean Shield had a hardware defect, and the transponder aboard the Bluefin might also have a defect, the Australian coordination center said.

Spare parts for both transponders have to be flown from Britain to Western Australia, and the Ocean Shield must return to port at Dampier. “The journey is anticipated to take a number of days,” the coordination center’s statement said.

Once the Ocean Shield reaches port, it will need to dock so that “engineers can make a full assessment of the transponder repairs.”

The delay in the deep-sea search is particularly ill-timed because Defense Minister Hishammuddin Hussein of Malaysia, who is also that country’s acting transport minister, said at a news conference in Kuala Lumpur on Thursday that Australia, Malaysia and China had agreed to end the air and sea surface search for Flight 370, which disappeared March 8.

The search effort will focus now on reanalyzing all of the data, including satellite transmissions and other information, that might provide clues to the aircraft’s final whereabouts, as well as comprehensive mapping of the seabed in the last suspected location in the Indian Ocean.

The Navy said that the cause of the communications defect could not be established until equipment was dismantled in port and the damaged components replaced and studied.

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