The investigation into the disappearance of flight MH370 is likely to turn into a criminal inquiry as mounting evidence points to a deliberate act, a leading air crash investigator said Wednesday.

With no distress call and no sign of a midair breakup, questions are now being asked about how the Malaysia Airlines Boeing 777 could apparently fly hundreds of miles off course without being tracked or detected by civilian radar systems.

“We don’t have much in the way of evidence so far, but what there is suggests a deliberate act,” said former National Transportation Safety Board (NTSB) investigator Greg Feith. "I think this will turn from an accident investigation into a criminal inquiry."

A deliberate act?

Feith said the apparent turn made by MH370 from its eastward flight path to a westward path suggests a deliberate action rather than just a drifting aircraft.

“If the military radar evidence is correct, what we see is not a slow, meandering curve but a deliberate turn followed by a straight line,” he said. “The plane doesn’t appear to be wallowing in the air, drifting left and right — it is being flown.”

Feith said the abrupt end of communications suggests the aircraft’s transponder was disabled deliberately rather than by a loss of power.

“If you lose power from the engines, you have backup generators. If you lose the generators, you have the auxiliary power unit. If you lose the APU, you have the ram air turbine — a sort of fan that drops from the fuselage and uses wind to generate power. And then you’ve got batteries. All these systems would provide power to the radio to contact air traffic controllers.”

So was the airliner hijacked, or did one or both pilots deliberately steer the plane off course?

“At this stage, there is simply no way of knowing,” Feith said.

A sudden, catastrophic failure or explosion would leave behind a trail of visible debris on the ocean surface.

But six days after the jetliner took off from Kuala Lumpur for Beijing, no trace of wreckage has been found, nor any signal from its emergency transponder beacon.

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Its last confirmed position on civilian radar was at 1:30 a.m Saturday (12.30 p.m. Friday ET) some 35,000 feet above the Gulf of Thailand between Malaysia and southern Vietnam. It sent no distress signals or any indication it was experiencing problems.

Malaysian authorities have since said that military radar detected what might have been the plane turning and flying west until it reached the Strait of Malacca — a busy shipping lane on the other side of the narrow country — some 250 miles from the plane's last known coordinates.

Malaysia’s air force chief, Rodzali Daud, told a news conference Wednesday that it had not yet been verified that the military radar data showed a Boeing 777. So the sea search continues on both sides of the country.

'Ghost plane' theory

With each day that passes without a sign of wreckage, theories involving mechanical failure are becoming less likely. One suggestion is that the plane suffered a slow decompression that starved the pilots of oxygen before they could descend to a survivable altitude.

That’s what caused the chilling crash in 2005 of Helios Flight 522, in which a pressurization failure turned a Boeing 737 into a "ghost plane." It continued on its computerized flight plan for more than two hours with pilots slumped over the controls before eventually running out of fuel and crashing into a Greek mountainside.

But experts believe this theory is unlikely to explain the mystery of MH370.

“If that had happened in this instance, the Boeing 777 would have continued on its flight plan toward China until air traffic controllers were unable to get a response from the pilots,” said U.K. crash investigator David Gleave. "At that point, fighter jets would be scrambled to intercept it. There would be no mystery over the plane’s whereabouts. And it doesn’t explain why there has been no sign of the Malaysia plane's emergency transponder."

Could the pilots have been trying to turn back when they ran out of oxygen, leaving the aircraft to drift westward?

“That’s also unlikely,” said Gleave. “The very first thing you would do is get your oxygen masks on and then descend as quickly as possible. You would only start making turns once you had got down to a lower altitude. And that descent would be picked up on the civilian radar.”

What about the possibility that someone invaded the cockpit and hijacked the plane?

“It might depend on whether Malaysia Airlines followed similar rules to U.S. carriers' that require the cockpit door to be locked from the inside at all times," said Feith.

On U.S. carriers, pilots going on a bathroom break must follow rules in which one flight attendant must take the place of the absent pilot while another blocks the forward galley using a cart so that no passengers could surge into the cockpit.

Eyebrows were raised Tuesday when the 27-year-old first officer of the missing jet was accused in an Australian news report of entertaining women in the cockpit on a 2011 flight — even allowing them to sit on the flight deck during takeoff and landing. The airline said it was “shocked” by the allegations.

Clues from past crashes

If suspicion turns to the pilots, there are some clues in earlier crashes such as EgyptAir Flight 990 off the coast of Massachusetts in 1999 and SilkAir Flight 185 over Indonesia in 1997.

U.S. officials who helped investigate the SilkAir crash found evidence consistent with a deliberate manipulation of the controls — most likely by the Singaporean captain, who had reportedly incurred debts of more than $1 million and was already facing disciplinary action.

The Boeing 737 went down just as it was between two air traffic control zones — an echo of MH370, which had been handed over from Malaysian controllers and was heading toward Vietnamese airspace.

In addition, the SilkAir jet’s cockpit voice recorder and flight data recorder had stopped working minutes before the abrupt descent, possibly the result of the captain's deliberately pulling a circuit breaker. A similar action would explain the failure of the emergency transponder on MH370.

But before any of these theories can be tested, investigators must first find the jet. If its westward trajectory is proved correct, it could have traveled for hundreds of miles.

“It could have kept flying for another three, four, five hours,” Feith said. “We don’t know exactly how much fuel was on the airplane. But if they’re saying seven hours [of fuel] — this turn comes about two and a half hours [into the flight], so really you have another five hours of fuel. So that plane could have kept going well into the Indian Ocean.”