SYDNEY, Australia -- Investigators believe the missing Malaysia Airlines plane flew on autopilot for hours before crashing into a remote part of the Indian Ocean, Australian officials said Thursday as they announced another shift in the search area for the jet.

After analyzing data exchanged between the plane and a satellite, officials believe Flight 370 was on autopilot the entire time it was flying across a vast expanse of the southern Indian Ocean, based on the straight path it took, Australian Transport Safety Bureau chief commissioner Martin Dolan said.

"Certainly for its path across the Indian Ocean, we are confident that the aircraft was operating on autopilot until it ran out of fuel," Dolan told reporters in Canberra, the nation's capital.

Asked whether the autopilot would have to be manually switched on, or whether it could have been activated automatically under a default setting, Dolan replied, "The basic assumption would be that if the autopilot is operational it's because it's been switched on."

But exactly why the autopilot would have been set on a flight path so far off course from the jet's destination of Beijing, and exactly when it was switched on remain unknown.

"We couldn't accurately, nor have we attempted to, fix the moment when it was put on autopilot," Transport Minister Warren Truss said. "It will be a matter for the Malaysian-based investigation to look at precisely when it may have been put on autopilot."

The latest nugget of information from the investigation into Flight 370 came as officials announced yet another change in the search area for the plane that vanished March 8 after taking off from Kuala Lumpur with 239 passengers and crew on board.

The transport safety bureau said it made the assumption in defining the new search area that the crew was unresponsive, possibly suffering from oxygen deprivation, as the plane flew under autopilot. It said this was indicated by the loss of radio communications and the long period without any manoeuvring of the plane. It emphasized, however, that this was only a working theory and did not mean that accident investigators led by Malaysia would reach a similar conclusion.

A loss of cabin air pressure could cause oxygen deprivation, or hypoxia, which could make pilots unable to perform even basic tasks.

The new search area is several hundred kilometres (miles) southwest of the most recent suspected crash site, about 1,800 kilometres (1,100 miles) off Australia's west coast, Dolan said. Powerful sonar equipment will scour the seabed for wreckage in the new search zone, which officials calculated by reanalyzing the existing satellite data.

The shift was expected, with Dolan saying last week the new zone would be south of an area where a remote-controlled underwater drone spent weeks fruitlessly combing 850 square kilometres (330 square miles) of seabed. That search area was determined by a series of underwater sounds initially thought to have come from the plane's black boxes. But those signals are now widely believed to have come from some other source.

The new 60,000-square kilometre (23,000-square mile) search area falls within a vast expanse of ocean that air crews have already scoured for floating debris, to no avail. Officials have since called off the air search, since any debris would likely have sunk long ago.

The hunt is now focused underwater. Beginning in August, private contractors will use powerful side-scan sonar equipment capable of probing ocean depths of 7 kilometres (4.3 miles) to comb the ocean floor in the new search zone. The job is expected to take 12 months to complete.

Two survey ships are mapping uncharted expanses of seabed in the search zone before the sonar scanning starts. Dolan said it was possible the mapping equipment could detect wreckage that may be lying on the seafloor, but that it was highly unlikely.

The search area has changed multiple times in the months since Flight 370 vanished, as officials struggled to make sense of the limited data the flight left in its wake after it dropped off radar. The plane's communications systems were disabled, giving investigators little to work with beyond data gleaned from hourly transmissions, or "handshakes," between the plane and a satellite.

University of New South Wales aviation expert Peter Marosszeky said if the autopilot was still working when the plane crashed, it suggests the aircraft's communications systems were switched off rather than disabled by a major malfunction or catastrophe.

"It would appear very unlikely that power was removed from most of the essential systems, because you can't connect your autopilot if your flight management computers aren't operating," he said. "It would appear that it lost all communication and identification with air traffic control because those systems were turned off. You can't connect the autopilot if you've got systems that have been put out of action."

John Cox, a Washington, D.C.- based aviation consultant, former airline pilot and accident investigator, said it was unclear at what point the autopilot was programmed to fly out into the open ocean.

But he said it was done by someone with expert knowledge.

"Someone with knowledge interacted with the flight management computer and told it to do certain things which requires knowledge of the proper entry key strokes to get it entered and to have the computer exercise those instructions," Cox said.

Truss said he was optimistic that the latest search zone is the most likely crash site. But he warned that finding the plane remains a huge task.

"The search will still be painstaking," he said. "Of course, we could be fortunate and find it in the first hour or the first day -- but it could take another 12 months."