Investigators looking into the disappearance of Malaysia Airlines flight MH370 have discovered a "mysterious 200lb load" added to the flight list after take-off, according to an engineer whose wife and two children were on board.

Ghyslain Wattrelos said the cargo was revealed in a report on the passengers and baggage by French investigators, the Telegraph reported.

Wattrelos, who believes the flight was deliberately downed, told Le Parisien newspaper: "It was also learned that a mysterious load of 89 kilos was added to the flight list after take-off. A container was also overloaded, without anyone knowing why. It may be incompetence or manipulation. Everything is possible. This will be part of the questions for the Malaysians."

MH370 became one of the world's greatest aviation mysteries when it vanished with 239 people on board en route from Kuala Lumpur to Beijing on March 8, 2014.

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French investigators who examined flight data at Boeing's headquarters in Seattle believe that the pilot was in control of the airliner "right up to the end".

The document from NNR Global does not specify how many lithium batteries were on-board. Photo / Supplied
The document from NNR Global does not specify how many lithium batteries were on-board. Photo / Supplied

Wattrelos said the investigators told him the data "lends weight" to the theory that the pilot crashed into the sea in a murder-suicide, although they stressed that there was no proof. The investigators expect it to take up to a year to examine the data fully.

However, some experts believe a hijack by a stowaway is a possibility and the mysterious load could lend credence to the theory. Tim Termini, an aviation security specialist, told Channel 5 earlier this month: "It's highly likely that a hijack took place and again, there's four options for the hijack.

"One is the hijack of the aircraft through a crew member. The second is a hijack coming from a passenger. A third option, which is a fairly unusual one, would be a stowaway. And then of course the fourth option is an electrical takeover of the aircraft from a ground-based station."

Wattrelos, 54, who has led a campaign to find out what happened to the flight, acknowledged that "there is a risk that I may never learn the full truth."

The plane was being piloted by Zaharie Ahmad Shah. Photo / Supplied
The plane was being piloted by Zaharie Ahmad Shah. Photo / Supplied

THE MAIN THEORIES OF MH370 DISAPPEARANCE

Since Malaysia Airlines Flight MH370 vanished in March 2014, while flying from Kuala Lumpur International Airport, Malaysia, to its destination, Beijing Capital International Airport in China, conspiracy theorists have speculated that:

1. It was shot down
According to this theory, Flight 370 could have been shot down during a joint military exercise between the United States and Thailand in the South China Sea. The theory is advanced in a book called Flight MH370: The Mystery by Nigel Cawthorne, a UK-based writer. The book describes the plane's disappearance as "the greatest mystery since the Mary Celeste".

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2. It was hijacked
With the 13th anniversary of the 9/11 attacks on New York and Washington approaching, conspiracy theorists have turned much of their focus to what some have dubbed the "9/11/14" theory. This theory, utterly devoid of proof but nonetheless widely circulated on the internet, suggests that Flight 370 did not crash but is in fact being prepared for use in a terrorist spectacular that will coincide with anniversary of the attacks on the World Trade Center and the Pentagon.

3. It was switched
The "plane switch" is among the most outlandish theories to have surfaced. One of its main backers is Jim Stone, a self-described "independent journalist" whose website carries the motto: "Truth is reality, which lies and inventions fall to in the end." Mr Stone proposes the theory that it was Malaysia Airlines Flight 370 and not Flight 17 that was shot down over the Ukraine in July last year. He offers little in the way of explanation about who might benefit from switching the planes, or how they might have achieved it.

4. It landed on an island
In the days and weeks following the plane's disappearance one of the most commonly heard conspiracy theories was that it had landed on Diego Garcia, the British-owned island in the Indian Ocean that is home to a major US military base. The theory gained so much traction that US officials were forced to deny it.

5. It was destroyed by a mysterious new weapon
Mike Adams, a veteran conspiracy theorist who runs a website called the Natural News, is known for his bizarre and often ludicrous articles. "There are some astonishing things you're not being told about Malaysia Airlines Flight 370," he wrote less than 48 hours after the disaster. His explanation for how MH370 "utterly and inexplicably vanished"? Not a hijacking nor a mid-air explosion but possibly the work of "some entirely new, mysterious and powerful force" capable of plucking "airplanes out of the sky without leaving behind even a shred of evidence".

6. Russia hijacked the plane
Under Putin's orders and flew it to Kazakhstan.

The reality check

Despite all those theories, the reality is certainly more mundane, says David Learmount, the aviation expert. He is convinced that whatever happened to the plane was "a deliberate act by someone on board, probably the captain". "I know people - especially journalists - would like this weird event to have more in common with Star Wars, but actually it was probably a carefully planned suicide and revenge attack," he said. "But will we ever be able to prove that? No."