The sudden disappearance of Malaysia Airlines flight MH370 on Saturday – with no distress call or other signs of trouble – has ignited intense speculation over what happened to the jet and its 239 passengers.
Following are some of the scenarios being mulled over by regional authorities, investigators and industry experts.
Q: Is mechanical or structural failure likely?
Sudden, accidental structural failures leading to explosions or a sudden loss of cabin pressure are considered extremely unlikely in today's passenger aircraft. This is especially so with the Boeing 777-200 model, which has one of the best safety records of any jet.
"From a crack, there can be a whole structure breakdown that allows for no response. But in the last two to three decades there have been next to nil such incidents," said Ravi Madavaram, an aviation analyst with Frost & Sullivan.
Indonesia-based aviation analyst Gerry Soejatman said based on the MH370 plane's maintenance records, "there is nothing that would jump straight out of the page".
Q: How likely is human error in this case?
The MH370 case may draw comparisons with the crash in 2009 of an Air France plane into the Atlantic Ocean, which killed more than 200 people. An investigation said speed sensors failed, causing the Airbus jet to stall and lose altitude. But it also said pilots failed to react correctly, losing control of the jet.
Soejatman said despite all the safety features on modern aircraft, well-trained pilots taking proper action in an emergency also is essential. "If the crew is not on the ball, they quickly lose control of the situation," he said.
Q: Was it an attempted hijacking or terror attack?
This spectre has loomed after authorities said at least two passengers had boarded with stolen passports. Malaysian officials also said on Sunday radar data indicated the pilot may have inexplicably tried to turn back to Kuala Lumpur.
Analysts said the absence of any distress signal raises eyebrows, as it could indicate an event so sudden that the crew could not respond. "There was not even time for the pilot or crew to raise an alarm. It could have happened due to a deliberate act – by a pilot or a terrorist – but this is all very speculative," Ravi said.
The terror theory's credibility is hurt by the fact that – so far – no claim of responsiblity has surfaced.
"What's the motive? If they didn't bring any weapons, it is extremely difficult to get into the cockpit," said Shukor Yusof, aviation analyst with Standard & Poor's.
He also noted that stolen passports do not necessarily equate to terrorism.
Large numbers of illegal workers, as well as criminal syndicates, are known to move between Malaysia and neighbouring countries such as Thailand. The two suspect passports were reportedly stolen in Thailand.
Q: Is lax security at Kuala Lumpur International Airport (KLIA) to blame?
The modern facility does not have a history of known security breaches. But Rohan Gunaratna, a terrorism expert at Singapore's Nanyang Technological University, said the passport issue could indicate a "glaring flaw" in the airport's immigration clearance.
He noted that Interpol maintains a database of stolen passports that should have raised red flags at the immigration counter. "There are two categories of people who use these (stolen passports) – criminals and terrorists," he said.
However, Shukor said the sheer volume of travellers moving through airports likely means not all forgeries can be caught. "To blame Malaysian authorities for this is probably unfair -- they have to get it right all the time and potential hijackers just have to get through once," he said.
Q: Could violent turbulence or bad weather have brought down the plane?
This possibility is being widely discounted as all indications are that the weather was fine in the area where contact with MH370 was lost.
Q: Could it have run out of fuel?
Malaysia Airlines has said the plane was fuelled for at least eight hours of flight. The Kuala Lumpur-Beijing route lasts six hours. Aircraft typically carry two hours' worth of fuel on top of what is needed.
Adds Ravi: "If there was a fuel loss, the pilot would have enough time to call for distress signal, and to turn around and glide back to land."