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But investigators do not believe the readings are accurate because the aircraft would most likely have taken longer to complete such a reduction in altitude. There is no explanation for intentionally flying such an erratic course, as it would harm the passengers and raise suspicion on radars.

The suggestion that a fire knocked out the transponders and caused the pilot to make a U-turn back to the nearest runway, before being overcome by smoke, leaving the plane flying on autopilot, was initially highly regarded. Some pilots still believe this is the most likely explanation. Others now say a fire large enough to take out the entire electrical system would make onward flight for seven hours impossible. They also say that there would usually be smoke before fire – allowing the pilots to put on oxygen masks and make a mayday call. But this could have proved difficult if there was rapid decompression.

Experts do say if the plane went down in the Southern Indian Ocean it is likely the plane was flying on autopilot, as the pilots were incapacitated in some way.

“The assumption is if you’re going off into the Southern Ocean, presumably the pilots were incapacitated by a fire or something, and it was flying on autopilot until the fuel ran out,” satellite communications consultant Tim Farrar told The New York Times. “That’s sort of implicit in the Southern Ocean assumption.”

Investigators appeared to use the plane’s final “ping” signals to a satellite to identify the search area, he said. They also likely assumed that the jet was flying at an undeviating speed toward Antarctica when it presumably plunged into the Indian Ocean, he said.

Sources told the BBC that flight MH370 continued to ping for at least five hours after the aircraft left Malaysian airspace – which indicated the plane was intact and powered.

Philip Baum, editor of Aviation Security International Magazine, said Monday’s announcement changes little.

“We still believe there was a deliberate act that took place on board the flight deck inside the cockpit that resulted in the aircraft turning and heading south,” he told the BBC.

Meanwhile, there was anger that after weeks of few answers from authorities, relatives of the passengers and crew were finally informed of the plane’s fate, either in person or via telephone. A text message was also sent.

Adrienne Mong of NBC News tweeted a screenshot of the text.