Efforts to divert the missing Malaysia Airlines plane needed "preparation and forethought" and appear to have been part of a hijack, aviation security experts have told Sky News.
While Malaysian authorities are refusing to say publicly that a hijack is the most likely scenario, the country's prime minister confirmed the plane had been deliberately diverted and its communications cut.
The Boeing 777-200 was heading from Kuala Lumpur to Beijing with 239 people on board last Saturday when it disappeared around 40 minutes into its flight.
And experts say the fact Flight MH370 vanished in a so-called 'dead space' area - when Malaysian air traffic controllers handed over control to their Vietnamese counterparts - appears a crucial factor in the mystery.
John Lindsay, former head of air safety at British Airways, said this would have been the "ideal" time to take over the aircraft "because it would give a period of time when no one was aware of what the aircraft was doing".
He said this and the disabling of the plane's transponders - which transmit data on a plane's location to air traffic controllers - suggest it was well-planned.
"It seems to be more than just a strong coincidence that the loss of contact with the aircraft happened at the point of hand-over," he told Sky News.
"(Also) there's a lot of (communication) equipment on there, most of which operates automatically, and to disable particularly the ACARS (Aircraft Communications Addressing and Reporting System) it would need some preparation and forethought.
"It's quite doable, but it's not something the pilots would have been trained to do, whereas the air traffic control transponder is something that is routinely switched on and off as required.
"But the ACARS are a different matter as are the sat coms. These things would have required some preparation and forethought about how they were going to be accomplished and when they were going to be accomplished."
He said it was "inconceivable" that someone in the passenger cabin would know the critical point at which the aircraft was being handed over to Vietnamese control.
This suggests either the collusion of the crew or someone in the flight deck when hand-over occurred.
Former BA pilot Alastair Rosenschein told Sky News it is "looking more likely" that the plane was hijacked.
"What is puzzling is the fact that the ACARS and the transponder appear to have been disabled or switched off or become unworkable at separate times," he said.
"It rather suggests that was a deliberate action."
Final satellite communication came more than six-and-a-half hours after the plane vanished from civilian radar.
Aviation security expert Philip Baum said the possibility of pilot suicide is now decreasing "because you would have expected a suicide pilot to simply crash the aircraft not carry on flying".
"So it now looks like we're dealing with a terrorist situation or with the actions of a deranged individual or an asylum seeker," he said.
Previous theories about why the plane vanished included a sudden mid-air explosion, catastrophic equipment or structural failure, or a crash into the South China Sea.
If a plane if hijacked, crew are trained to be compliant and do whatever they can to safeguard the plane and passengers.