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The last hope for families who have endured five years of anguish and uncertainty since Malaysia Airlines flight MH370 disappeared with 239 people on board may lie with a small team of French investigators.

France is the only country still conducting an official inquiry into one of aviation's most enduring and fascinating mysteries: why did the Boeing 777 disappear en route from Kuala Lumpur to Beijing and where did it end up?

Theories abound about what happened, but there is still no definitive answer. The most extensive — and expensive — search in aviation history failed to recover the aircraft or its black boxes.

About 30 pieces of suspected plane debris have been found washed up on beaches thousands of miles apart in the Indian Ocean. Some have been confirmed as having come from MH370.

There has been speculation that the airliner was hijacked. A more plausible theory, according to investigators in the aviation and intelligence communities, is that the captain decided to carry out a murder-suicide by changing course to fly south until the plane ran out of fuel and crashed into the Indian Ocean.