Australian authorities spent more than $160million over three years in search of answers as to the Boeing 777’s mysterious disappearance – but no definitive answer has ever been disclosed. Data from the flight, as well as plane debris washed up on beaches thousands of miles apart, have failed to account for the plane’s disappearance. But a recent French investigation, the only official inquiry still being conducted, appears to lend weight to an existing theory – which could reveal what really happened to flight MH370, en route from Kuala Lumpur to Beijing.

French authorities are leading a judicial inquiry, as four French citizens were onboard the flight, and despite the investigation being protected under strict confidentiality laws, many aspects have been leaked to the media.

Some information has been provided by Ghyslain Wattrelos, a 54-year-old French executive whose wife and two of his children were on board the flight.

His latest revelations suggest investigators suspect the pilot remained in control of the plane until the end, The Daily Telegraph reports.

This revelation supports the murder-suicide theory, where Captain Zaharie Ahmad Shah decided to change course to fly south until the plane ran out of fuel in order to carry out a mass murder and suicide.

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The theory, posited as the most likely explanation by investigators in aviation and intelligence communities, suggests the pilot made a U-turn to fly back over Malaysia heading towards the Indian Ocean.

To stop fellow crew members or passengers putting up resistance, it is thought he starved them of oxygen, causing them to die in a matter of minutes.

But this idea is at odds with the official theory of the Malaysian and Australian investigators that MH370 was a “ghost flight” at the end, with the pilot presumed dead after the cockpit became depressurised before the plane ran out of fuel and plunged into the sea.

Michael Exner, an electrical engineer, supports the murder-suicide theory but he said no one can be sure whether the pilot was alive or dead at the end.

He said radar data indicated the plane climbed 40,000ft as it made its U-turn.

Mr Exner said: “We all concluded that this is 100 percent inconsistent with any scenario of an accident or engine failure.

“We know it was deliberate.”

He believes the plan passed Kota Bhara, near the Thai border, at an altitude of 40,000ft – which could have been to accelerate the effect of depressurising the cabin, to kill its occupants by starving them of oxygen.

If this did happen, drop-down oxygen masks would have appeared, but as they are designed for emergency descents to below 13,000ft they would have been useless at 40,000ft.

As a result, passengers would have become incapacitated within a few minutes, lost consciousness and died without choking or gasping for air.

It is suggested the pilot’s cockpit companion, First Officer Fariq Hamid, could have been in the passenger cabin during this time.

Can Higgings, an Australian journalist who wrote a book about the plane’s disappearance believes the theory the plane was unpiloted at the end may be politically motivated.

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He said Australian investigators, possibly influenced by the Australian government who wanted to maintain good relations with Malaysia, “didn’t want to get into a position where they had to say a Malaysian national, a Malaysian pilot, had taken 238 people to their deaths”.

The mass murder-suicide theory isn’t the only one to gain traction, with speculation that the airline was hijacked.

But Malaysian investigators found no evidence that the aircraft was flown by anyone other than the pilots, but did not rule out “unlawful interference by a their party”.

Other theories suggest the plane was hit by a missile as it was on course for a 9/11 style attack.