Malaysia's acting Transport Minister Hishammuddin Hussein shows two maps with corridors of the last known possible location of the missing Malaysia Airlines MH370 plane as he addresses reporters at the Kuala Lumpur International Airport in this March 17, 2014 file photo.[Photo/Agencies]

It was accidentally shot down

This theory was the thrust of the first book published on the incident, 'Flight MH370 The Mystery'. London-based author Nigel Cawthorne said the plane may have been accidentally shot down during joint US-Thai military exercises in the South China Sea. Such accidents have happened before: Korean Air flight 007 was shot down by the Soviet Union in 1983, and the US Navy downed an Iranian airliner in 1988. Aviation experts are sceptical about a US and Thai cover-up, proponents argue the very nature of a "cover-up" is that it is hard to disprove.

It was brought down near Diego Garcia 

The former head of France's Proteus Airlines, Mac Dugain, put forward the theory that the plane may have been shot down by the US military, fearing a Sept. 11 style attack on the US Navy base on the remote Indian Ocean island of Diego Garcia. Dugain pointed to the testimony of residents in the Maldives who reported seeing an airliner travelling towards the island, although their claims were widely dismissed.