As the hunt continues for missing Malaysia Airlines flight 370, Channel 4 News looks at four plane crash mysteries which kept investigators scratching their heads for years.

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Egypt Air Flight 990

On 31 October 1999, the Egypt Air Flight 990, a Boeing 767, crashed into the Atlantic Ocean about 60 miles south of Nantucket Island, Massachusetts, killing all 217 people on board. In 2002 the US National Transportation Safety Board ruled that pilot error was the cause of the crash.

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Air France 447

The Air France 447 flight from Rio de Janeiro to Paris that crashed in 2009 plummeted 38,000ft in just three minutes and 30 seconds because pilots lost vital speed data. The pilots on the aircraft got conflicting air speeds in the minutes leading up to the crash, a report found in 2011.

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Pan Am Flight 103

Pan Am Flight 103 was a transatlantic flight from Frankfurt to Detroit via London and New York City that was destroyed by a terrorist bomb on 21 December 1988, killing all 243 passengers and 16 crew on board. Large sections of the aircraft crashed into Lockerbie, Scotland, killing 11 more people on the ground.

Following a three-year joint investigation, Libyan intelligence officer Abdelbaset al-Megrahi was jailed for the bombing. He was released from jail by the Scottish government on compassionate grounds after being diagnosed with prostate cancer and died in 2012.

On Tuesday former Iranian intelligence officer Abolghassem Mesbahi told an Al Jazeera documentary that the bombing was ordered by Iran in retaliation for a US strike on an Iranian passenger plane.

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TWA Flight 800

TWA Flight 800 fell out of the sky on 17 July 1996, shortly after leaving JFK International Airport, killing all 230 people on board. Some eyewitnesses claimed that they saw a missile strike it in mid air.

A report in 2000 said that the probable cause of the accident was an explosion of flammable fuel/air vapours in a fuel tank, and, although it could not be determined with certainty, the most likely cause of the explosion was a short circuit.