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CANBERRA, Australia — Nearly four months after Malaysia Airlines Flight 370 vanished, Australian transportation officials said in a report Thursday that the plane kept flying until it ran out of fuel, most likely because the cockpit crew had become unresponsive because of oxygen deprivation.

The plane appears to have flown in a straight line south across the Indian Ocean controlled entirely by the autopilot, Australian officials said.

But they avoided offering hypotheses for why the plane had reached the northern end of the Indonesian island of Sumatra on March 8 and had turned south in the first place, when it was supposed to go from Kuala Lumpur, the Malaysian capital, to Beijing.

Malaysian radar records show instead that the Boeing 777-200, with 239 passengers and crew on board, did a U-turn over the Gulf of Thailand, banked right across the Malaysian Peninsula and banked right again to reach the northern tip of Sumatra.

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Evidence of an unresponsive crew as the plane flew south for more than five hours includes the loss of radio communications, a long period with no maneuvering of the aircraft, a steadily maintained cruise altitude and eventual fuel exhaustion and descent, the report said. It added that this could have been caused by hypoxia, or oxygen deprivation, among the cockpit crew.

“Given these observations, the final stages of the unresponsive crew/hypoxia event type appeared to best fit the available evidence for the final period of MH370’s flight when it was heading in a generally southerly direction,” the document said.

Hypoxia occurs when a plane loses air pressure and the pilots, lacking adequate oxygen, become confused and incapable of performing even basic manual tasks, although they continue to feel confident in their own abilities.

Pilots are trained to put on oxygen masks immediately if an aircraft suffers depressurization; their masks have only an hour’s air supply, however. The plane turned south toward the Indian Ocean about an hour after it stopped responding to air traffic controllers. It is believed to have crashed in the southern Indian Ocean off the western coast of Australia.

Passengers’ masks have only a few minutes of oxygen, based on the theory that the pilot of a troubled plane will quickly descend to an altitude at which there is little need for a supplemental air supply.

There is no consensus among investigators on the hypoxia or unresponsive-crew theory. Angus Houston, retired head of the Australian military who is overseeing the country’s search, said in a telephone interview this month that he assumed the flight had been on autopilot even if a conscious pilot had been at the controls. That is because a Boeing 777 is a very difficult plane to fly manually.

Other officials, who insisted on anonymity because of the diplomatic sensitivity of the issue with Malaysia and China — most of the flight’s passengers were Chinese — said some investigators still leaned toward the possibility that one of the pilots flew the plane to the southern Indian Ocean in a suicide mission that also killed everyone else aboard.

At a news conference in Canberra, Martin Dolan, chief commissioner of the Australian Transport Safety Bureau, said someone on the plane had put it on autopilot, but he declined to speculate as to who might have done so and why.

Based on recent analysis of data from electronic “handshakes” between the plane and a satellite operated by the company Inmarsat, the 777 appears to have followed a straight track to the south, Dolan added.

Warren Truss, the deputy prime minister of Australia and the minister for infrastructure and development, said at the news conference that Australia planned to hire a contractor to scour a rectangular area of ocean floor covering 23,000 square miles. Up to three deep-sea submersibles will be used for the yearlong endeavor, starting in August.

The midpoint of the new search area is 1,100 miles west-northwest of Perth, Australia.

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