Australian officials say they are confident the missing Malaysian jet was on autopilot when it crashed and that the plane's pilots were likely unresponsive due to a lack of oxygen.
The comments Thursday are the firmest theory yet offered by officials on what may have led to the Boeing 777's disappearance, which has become one of modern aviation's greatest mysteries.
At a news conference, Deputy Prime Minister Warren Truss said it is "highly, highly likely" the plane was on autopilot when it ran out of fuel. Otherwise, he says the aircraft would not have followed such an orderly path seen in the satellite readings used to track its likely resting place.
In a report issued later Thursday, the Australian Transport Safety Board said the plane's steady flight path and lack of communications lead it to believe that a "hypoxia type event appeared to fit the best available evidence."
Hypoxia, which can occur when a plane loses air pressure, is caused by a lack of oxygen to the brain, resulting in confusion, unconsciousness, and eventually death. Officials and analysts have long said depressurization is one of many possible scenarios being investigated.
The Malaysia Airlines Boeing 777 was headed from Kuala Lumpur to Beijing on March 8 when it disappeared with 239 people on board. It did not send a distress call.
The plane stopped communicating with air traffic controllers in the Gulf of Thailand. It then inexplicably turned west and headed for the Strait of Malacca before heading south, after which investigators believe it crashed in the southern Indian Ocean.
Since the jet's communications devices were either shut off or disabled, authorities have had to estimate the plane's final resting place by conducting a complex analysis of data sent from the plane to a communications satellite.
On Thursday, Deputy Prime Minister Truss said a "more thorough" analysis of that satellite data has led investigators to shift the search area south of where an initial underwater hunt took place.
"The new priority area is still focused on the same seventh arc in the southern Indian Ocean, where the aircraft last communicated with satellite. We are now shifting our attention to an area further south along that arc, broadly in the area where our first search efforts were focused. The area has already been subject to aerial and visual searching for wreckage and debris, but now we will move to an underwater search," said Truss.
The new area to be searched is a 60,000 square kilometer arc in the southern Indian Ocean, about 1,800 kilometers off the southwest Australian coast.
Ships are currently carrying out a complex, bathymetric survey in the area to learn more about the depths and shapes of the underwater terrain.
The process will take around three months. Officials then expect the underwater search to begin in August and take up to 12 more months.
An underwater drone recently finished up an unsuccessful, two-month-long search of 850 square kilometers of Indian Ocean seabed.
That location was near where authorities heard faint electronic signals thought to be from the missing jet's flight data recorder.
Authorities now conclude they were likely distracted by the "pings," which turned out to be a false lead.
Malaysian authorities, who believe someone intentionally diverted the plane, have not commented on the findings announced Thursday by the Australian government.