When Malaysian authorities disclosed this weekend that flight MH370 was deliberately steered off course and the equipment sending out its location disabled (or the subject of an unlikely malfunction), attention focused on the Boeing 777’s pilots.
Captain Zaharie Ahmad Shah, 53, the "loving and generous" father of three, reportedly travelled on the day of the flight to the court where Malaysian opposition leader Anwar Ibrahim was jailed on what his supporters say were trumped-up sex charges. Shah was a committed supporter of Ibrahim but his friends have denied he was a "fanatic".
The theory that the fate of the plane was sealed from within its cockpit also raises questions of the second pilot, Fariq Abdul Hamid, 27, who was yesterday confirmed as saying "all right, good night" when the aircraft signed off from Malaysian air traffic control for the last time. He posted the online comment "Time to take passion to next level" before the flight.
Searches of both men's homes, including the confiscation of Capt Shah's flight simulator, have revealed no obvious clues such as a suicide note. But there are precedents for pilots destroying their own aircraft. In a finding disputed by the Egyptian authorities, it was concluded that EgyptAir flight 990 crashed into the Atlantic in 1999 as a result of the actions of co-pilot Gameel al-Batouti, killing all 217 on board.
Whoever may have turned off MH370's location technology - the transponder that interacts with radar and the Acars maintenance computer which sends data via satellite - knew how to make an aircraft disappear from most tracking systems. Anonymous US officials have advanced the theory this could be evidence of an "act of piracy".
When combined with the idea that the Boeing 777 could have descended to an altitude of 5,000ft and undertaken the sort of " terrain masking" manoeuvres more normally associated with agile military jets, the idea is born that MH370 is sitting on an unknown runway with its passengers and crew held hostage.
The idea that one of the "pings" received by satellites after the loss of contact with the jet could have been from the ground has fuelled this particular area of speculation.
The Pakistan Taliban said on Monday it had nothing to do with the disappearance of MH370 but wished it had the resources and knowledge to hijack such a plane. The existence of a bomb on board the plane or a hijacking ending in a fatal crash cannot be excluded. But intelligence officials say there has been no "chatter" in extremist circles.
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Ordinary lives of flight crew
A rogue passenger
Malaysian police are investigating Mohd Khairul Amri Selamat, a 29-year-old aviation engineer flying from Kuala Lumpur to Beijing to carry out work on a plane in China. As a flight engineer Selemat, would have had some of the technical knowledge required to fly an aircraft. But the Malaysia-based technician would not have had knowledge specific to the Boeing 777.
Mechanical failure or pilot error
72 per cent of air disasters are caused either by mechanical failure or a mistake by the pilot. Sudden, catastrophic failure is diminished by the evidence that the plane changed direction and continued flying for several hours.