The missing Malaysia Airlines flight MH370 flew on autopilot until it ran out of fuel above the southern Indian Ocean, with its crew likely "unresponsive", experts believe.
The latest theory on the fate of the plane - which disappeared more than three months ago - came as Australia announced that the underwater search for the jet would shift further south.
Deputy Prime Minister Warren Truss said yesterday that an expert review had found it was "highly, highly likely that the aircraft was on autopilot" when it crashed. "Otherwise it could not have followed the orderly path that has been identified through the satellite sightings," he said.
The jet, with 239 people - mostly Chinese - on board, disappeared on March 8 on its way from Kuala Lumpur to Beijing.
An extensive Australian-led search has so far failed to find any trace of the jet. New search efforts will now concentrate on an area 2,000 kilometres off Australia's west coast near where the original search began in late March.
The new search area, covering up to 60,000 square kilometres, is along the so-called seventh arc, one of several possible routes projected by investigators. Starting in August, the seabed search at depths of up to 5,000 metres will last up to a year.
"We are now shifting our attention to an area further south ... broadly in the area where our first search efforts were focused," Truss said.
Australian Transport Safety Bureau chief commissioner Martin Dolan said the plane's journey over the Indian Ocean was largely guided automatically, giving rise to the likelihood that the crew had been rendered unconscious.
Theories on the plane's fate have included hijacking, a rogue pilot and mechanical failures.
But the ATSB report released yesterday said the most likely scenario was the crew suffering from hypoxia, or lack of oxygen, possibly from the plane losing air pressure at high altitude.
The report said "the hypoxia event type appeared to best fit the available evidence for the final period of MH370's flight".
Graham Edkins, a former ATSB safety investigator, said the time and distance for which the Boeing 777's autopilot function was used was not considered "unusual".
He supported the idea that the crew was "incapacitated", adding that it did not necessarily "indicate there was some kind of terrorist activity".
Malaysia last week denied claims that experienced Boeing 777 captain Zaharie Ahmad Shah was a prime suspect in investigations surrounding the flight's disappearance.
Additional reporting by Agence France-Presse