The theory that Malaysia Flight 370 flew for hours into the southern Indian Ocean on autopilot — the “zombie flight” theory — has gained considerable new credence. The Australian Transport Safety Bureau now says that it is fixing the new area to be searched for the remains of the Boeing 777 on the assumption that the airplane maintained its cruise altitude and speed until it ran out of fuel.

Significantly, in the last week the Australians have brought a lot more clarity to the behavior of the airplane after looking at the available data. They refuted the earlier readings of Malaysian radar that the 777 had made extreme changes of altitude, climbing to 45,000 feet before diving below 23,000 feet. They said Malaysian radar, and other military radars in the area, were not capable of giving an accurate picture of the height of the airplane as it changed course and made a sharp left turn over the Gulf of Thailand toward the Indian Ocean.

At a news conference in Canberra, the Safety Bureau’s chief commissioner, Martin Dolan, said that the airplane’s autopilot had been switched on but he declined to speculate by whom and when that had happened. The result was that the 777 continued for the rest of its life flying as any airliner would during a trans-oceanic flight, at normal cruise height and at normal cruise speed with all its systems functioning normally—except that there were no signs of life in the cockpit, no human communications.

This was the scenario I advanced two weeks after the 777 disappeared in March. And it’s important to realize that it does not explain why this was so—only that, given that it happened—and nothing like it has ever happened to a commercial flight before—all attention should focus on what might have overtaken the crew as they departed their flight path to Beijing.

In short, what (or who) incapacitated the crew?

On the basis of their own analysis of Flight 370’s flight path, the Bureau’s new report finds it plausible that the crew (and passengers) suffered a loss of oxygen, causing the condition known as hypoxia. This would be caused by a loss of pressurization, some kind of rupture to the airplane’s structure that results in oxygen masks being deployed for passengers and crew.

There are three kinds of decompression events that can effect an airliner at cruise altitude: an explosive decompression so extensive and sudden that the airplane is instantly destroyed; a rapid but non-explosive decompression in which the pilots are able to dive down to around 10,000 feet to recover (where the levels of pressure inside and outside the airplane begin to come back into balance); or a slow decompression in which a small leak, hard to detect at first, slowly depletes the supply of oxygen and leads to hypoxia.

It would have to be the last of these three that struck Flight 370—if, indeed, it was a case of depressurization.

There are at least two problems with this proposition though.

The first is that the pilots’ oxygen masks have microphones. They should have been able to report what had happened. And yet….

The clearest precursor of this scenario was Helios Airways Flight 522 flying from Cyprus to Athens in August 2005. It suffered a pressurization leak in a cabin door and the pilots failed to detect its effects, had not deployed oxygen masks, and lost consciousness.

Frequent attempts to contact the Boeing 737 failed as it flew on under the control of the autopilot. A male flight attendant, remarkably, regained consciousness and tried to save the airplane but lacked the skills and as the engines ran out of fuel it crashed, killing all 121 people on board.

However, there is a big difference between the circumstances of the Helios Airways 737 and Flight 370—the 737 was flying a short, well-traveled route in daylight, being tracked by controllers and radar. Flight 370 flew for as long as seven hours, most of it well beyond radar range, over an ocean otherwise empty of airplanes and at night—against a background of confusion among controllers and a long delay in recognizing that the 777 had gone rogue.

Moreover, there is no previous reported emergency aboard a 777 on a commercial flight that involved decompression. (There were, however, two emergencies on 777 test flights in 1995 where a flaw in an air conditioning unit caused rapid decompression and the pilots had to make extremely skillful emergency landings.)

The second problem is that depressurization could not have caused the initial loss of automatic communications between the 777 and the ground — constant signals from its transponder giving its position and half-hourly data bursts from its ACARS system that monitors its systems.

Could this, in fact, be a coincidence of human intervention followed by mechanical failure?

Some airline chiefs believe that there was an invasion of the cockpit. Tim Clark, the boss of Emirates Airlines, which operates the largest 777 fleet, has said that the airplane’s links to the ground were expertly terminated by somebody with a far deeper knowledge of its systems than any of his own pilots.

But if this were so, why was the rest of the flight left to the autopilot and no threats or declaration of a hijacking broadcast from the cockpit? What would be the point of just making an airplane disappear without trace?

There is one speculative answer: that at some point shots were fired and a bullet went through the fuselage structure in such a way as to cause a slow decompression. That would have incapacitated both perpetrators and crew.

In any event, depressurization may be a credible explanation but not one that eliminates all other possibilities. The Australians made it clear that they are not ruling out anything—without physical evidence from the wreck they could hardly do so.

I have previously drawn attention to the cargo aboard Flight 370, which included a large consignment of lithium-ion batteries. There has so far been no adequate public audit of this cargo, no record of who decided that the packaging of the batteries did actually meet international standards (as the cargo manifest claimed), what level of inspection was carried out as it was loaded to ensure that the consignment was not damaged, no precise details on its location in the cargo hold, and no statement that the airplane’s fire suppression and smoke detection systems would have been able to deal with combustion in the batteries.

There are altogether too many unknowns about both the specific condition of this cargo and the consequences to an airplane—any airplane—of lithium-ion batteries in cargo discharging gases as the result of a thermal runaway. While this is the case, the possibility that fumes from the cargo hold incapacitated the passengers and crew (and that a brief, localized fire could have disabled the communications circuitry) should be given as much weight as the possibility that the cause was depressurization.

Finally, there is a basic truth that must be used to test any theory: The airplane itself had no defenses against whatever caused the pilots to cede control to the autopilot. That means no defense against both mechanical failure and human intervention.