What could have happened to Malaysia Airlines flight MH370?

Six days into the search for Malaysia Airlines flight MH370, there is still no trace of the plane or the 239 people who were on board when it vanished on Saturday.

Here are some of the possible scenarios being weighed up by industry experts as the world waits to discover the fate of the Boeing 777, which has one of the best safety records of any jet.

THEORY: Explosion on board

WHY: Malaysian authorities say the plane vanished en route from Kuala Lumpur to Beijing without making a distress call, in clear weather, pointing to the possibility of a sudden catastrophic event.

The presence of two passengers who were travelling on stolen passports fuelled early fears of a terrorist attack. Authorities now believe the two Iranian men were simply illegal migrants, but CIA director John Brennan has said a terror attack has not been ruled out.

Other possibilities include a strike by a missile or military aircraft.

EXPERT VIEW: Analyst Ravikumar Madavaram said data from the plane's communications systems could prove crucial in ruling out a sudden catastrophe.

"If (all the systems) went off at the same time, it points towards a mid-air blast," said Ravikumar, an aerospace and defence consultant at Frost & Sullivan Asia-Pacific.

"But if there is a significant time lag and there are signs the systems were shut down sequentially, definitely that goes towards the theory about hijacking or a sabotage by the pilots themselves."

Unconfirmed reports say the two main communication systems shut down 14 minutes apart, which would not suggest an explosion.

But Australian aviation expert Neil Hansford said that for him, the available evidence still pointed to sudden catastrophe.

"I believe it's been hit with a military aircraft or some military ordnance, a stray missile or something similar," he said, adding that a bomb was another possibility.

THEORY: Hijacking

WHY: Authorities and media reports have repeatedly suggested the plane may have veered sharply away from its flight path. On Friday the hunt spread to the edges of the Indian Ocean -- a huge distance from the original search area.

The idea that the plane could have flown for hours in the wrong direction could lend credence to the theory of a cockpit takeover.

But analysts say the plane would surely have been picked up by regional military radars if it strayed so far off-route, noting that Malaysia, India, Thailand and Indonesia all monitor the area in question.

EXPERT VIEW: "The burning question is, how many military radars would the aircraft have gone through to reach the Indian Ocean?" said Gerry Soejatman, an aviation analyst based in Jakarta.

"How could it get past all of that? And if it did, how many people in the military are going to lose their jobs?"

If the plane did fly onwards, there is also the pressing question of whether it was hijacked or changed course for another reason.

Frost & Sullivan's Ravikumar said he believed several factors rule out a hijacking -- including the lack of a credible claim of responsibility.

THEORY: Technical difficulties

WHY: The sudden disappearance could also point to a technical problem forcing a rapid descent.

The reports that the jet may have veered west before losing contact could also point to the pilots struggling to rectify a problem.

EXPERT VIEW: "To me (the veer) suggests there was a stall," said Mary Schiavo, an aviation lawyer and inspector general of the US Department of Transportation.

"That doesn't mean you lose your engines. It means that you're losing your air flow over your wings, sufficient speed to keep the plane in the air," she told Australia's ABC television.

"It would lose altitude really dramatically."

She compared the possible scenario to the fate of Air France 447, which crashed into the Atlantic Ocean in June 2009 after its speed sensors malfunctioned and the pilots lost control.

THEORY: Structural disintegration

WHY: The lack of wreckage or black box transmission has led to speculation that the plane may have disintegrated in mid-air.

On Wednesday it emerged that US regulators warned months ago of a "cracking and corrosion" problem on Boeing 777s beneath their satellite antenna that could lead to a mid-air break-up.

But Boeing later confirmed that the warning did not apply to the missing plane, which had a different kind of antenna.

EXPERT VIEW: While structural disintegration has been behind some previous aircraft disappearances, new planes use "better materials, technology and maintenance schedules", Ravikumar said.

"This last happened to China Airlines flight 611, during its cruise at 35,000 feet in 2002. Flight 611 was a Boeing 747 aircraft and the reason for that crash was faulty repair."

He noted that the technology on a Boeing 747 was 20 years older than on a 777.

THEORY: Pilot suicide

WHY: While rare, there have been cases in the past of pilots crashing planes to take their own lives. In December 1997, a SilkAir Boeing 737 from Jakarta to Singapore plunged into a river in Indonesia with the loss of 104 passengers and crew. US investigators blamed pilot suicide.

EXPERT VIEW: A suicide bid "is possible and if that's the case there might not be a lot of debris because the plane would have come down in relative structural integrity", said Terence Fan, aviation expert at Singapore Management University.

"The airplane is not meant to float and if the airplane sinks in the water, water will go inside because the door seals are not meant to seal water."

Nothing has emerged to suggest any serious psychological problems with either of the pilots who were flying MH370, although an Australian TV report has accused the first officer of violating airline rules in 2011 by allowing two young South African women into the cockpit during a flight.

While theories abound as to the fate of flight MH370, Malaysia is not ruling anything out as the second week of searching approaches.

"Anything is possible," Transport Minister Hishammuddin Hussein said Friday.