1. Act of terrorism
There was speculation that MH370 might have been attacked by terrorists after the Malaysian authorities said on Sunday they were investigating two passengers who were using stolen passports.
But officials and experts was quick to point out that there was no proof of foul play so far and there could be other explanations for the use of false identity documents.
The two passengers bought their tickets through China Southern Airlines, which was code-sharing the flight with Malaysia Airlines. They were using the documents of an Italian and an Austrian who apparently had their passports stolen in Thailand during the past two years, and had made police reports about the theft.
Mr John Goglia, a former board member of the National Transportation Safety Board, the US agency that investigates plane crashes, said the lack of a distress call from the pilot suggested that the plane either experienced an explosive decompression or was destroyed by an explosive device.
"It had to be quick because there was no communication," Mr Goglia told Reuters, adding that the false identities of the two passengers was "a big red flag".
Mr Kip Hawley, a former administrator of the US Transportation Security Administration, told Bloomberg News that the incident "sounds like a lot of other plots". He referred to an incident in 2006 involving terrorists who wanted to down jetliners in the Atlantic Ocean by using liquid explosives. That plan was foiled by United States and British officials. Mr Hawley told Bloomberg News that he has been especially concerned about bombs hidden in the shoes of passengers as they are powerful enough to bring down aircraft.
According to a 2012 report by the US State Department, Malaysia has been vulnerable to terrorist activity and it has been used as a transit and planning hub for terrorists. Still, the department noted that the country has not suffered a serious terrorism incident for "several years".
Others noted that the plane's disappearance came at the end of the annual National People's Congress in Beijing, and took place at a time of rising concern in China about terrorism.
Colonel Colonel Richard Kemp, former counter-terrorism head of the British government's Joint Intelligence Committee, told British papers that the possibility of a terror attack has to be seriously considered. He pointed to reported links between between the separatists from China's Xinjiang and Al-Qaeda. China has blamed the separatists for a recent attack in Yunnan which left 29 people dead.
Mr Steve Vickers, the chief executive of a Hong Kong-based security consulting company that specialises in risk mitigation and corporate intelligence in Asia, told The New York Times that the presence of multiple travellers on stolen passports aboard a single jet was rare and a potential clue.
But some have warned that it was too early too jump to conclusion because forged travel documents were also used routinely by smugglers and illegal immigrants.
A US Department of Homeland Security official told The Los Angeles Times: "Just because they (the passports) were stolen doesn't mean the travellers were terrorists. They could have been nothing more than thieves. Or they could have simply bought the passports on the black market."
The Los Angeles Times also quoted a top federal law enforcement official in Washington as saying that no known terrorist link has surfaced, and no organisation has claimed responsibility for downing the plane.
Some noted that there have been cases of plane hijacking. But experts said it seems unlikely in this case given that the hijackers would typically force the plane to land at an airport and make known their demand. But some said a Sept 11-like hijacking is possible, with terrorists forcing the plane into the ocean.
2. Explosion on board
Some pilots and aviation experts said an explosion on board appeared to be a likely cause. The plane was at cruising altitude, the safest phase of flight, and likely would have been on autopilot.
"It was either an explosion, lightning strike or severe decompression," said a former Malaysia Airlines pilot who declined to be named. "The Boeing 777 can fly after a lightning strike and even severe decompression. But with an explosion, there is no chance. It is over."
Others said an extreme, sudden loss of cabin pressure could have caused an explosive decompression and broken the plane apart. Such a decompression can be caused by corrosion or metal fatigue in the airframe.
3. Mechanical fault
It emerged on Sunday that the plane had made an unusual U-turn attempt in the final moments before radar contact was broken off, triggering speculation that it could have turned back because of mechanical fault.
Experts said an "air turn back" or ATB means the aircraft has to return to the airport of origin as a result of a malfunction or suspected malfunction of any item on the aircraft. But they noted that the pilot would have made a distress call or signal about the turn back.
Malaysia Airlines chief executive Ahmad Jauhari Yahya added that the Boeing 777's systems would have set off alarm bells. "When there is an air turn-back, the pilot would be unable to proceed as planned," he said.
Some also raised the possibility that both engines of the plane could have failed.
In January 2008, a British Airways 777 crashed about 1,000 feet short of the runway at London's Heathrow Airport. As the plane was coming in to land, the engines lost thrust because of ice buildup in the fuel system. There were no fatalities.
Experts said loss of both engines is possible in this case, but they also noted that the plane could glide for up to 20 minutes, giving pilots plenty of time to make an emergency call.
In January 2009, a US Airways A320 lost both of its engines after taking off from LaGuardia Airport in New York. But the captain still had plenty of communications with air traffic controllers before ending the six-minute flight in the Hudson River.
Reports on Sunday said that the Malaysia Airlines plane suffered a broken wing tip when it was involved in a minor collision with another aircraft in 2012. But the airline said the damaged portion of the wing tip, approximately a metre, has been repaired by Boeing.
4. Sudden stall of the plane
Some experts have pointed to some similarities between the MH370 mystery and the the loss of Air France Flight 447, which crashed in the Atlantic Ocean in 2009 en route from Rio de Janeiro to Paris, killing all 228 passengers and crew on board.
The Air France accident was initially blamed by the airline on a thunderstorm. Later, investigators found that speed sensors, known as pitot tubes, on the outside of the Airbus iced over and caused the auto-pilot to disengage.
But data recovered after a two-year search led the authorities to conclude that pilot error had also played a part - the crew's handling of the plane after the auto-pilot was disengaged put it into a stall from which it could not recover.
A stall is when a plane stops flying and starts falling. According to investigations, the pilots - who had never been trained to fly the aircraft in manual mode at high altitude - had raised the nose of the plane repeatedly when they should have been lowering it, thereby bleeding off speed and eventually putting the plane into a stall.
5. Pilot fault
Some said pilot disorientation could be a possible - though unlikely - cause.
The pilots could have taken the plane off autopilot and somehow went off course and did not realise it until it was too late. But some experts pointed out that this was unlikely because the plane probably would have been picked up by radar.
There have also been rare cases of pilot suicide.
A Cairo-bound EgyptAir flight, which took off from New York in 1999, crashed into the Atlantic south of Massachusetts, killing all 217 passengers and crew.
The US National Transportation Safety Board ruled that the crash was caused by co-pilot Gamil el-Batouty. According to reports, he had deliberately crashed the plane as an act of revenge after he had earlier been reprimanded for sexual misconduct and the executive who told him he would not be allowed to fly US routes again was on board the plane.
The Egyptian authorities, however, disagreed with the cause of the crash, blaming it instead on technical problems.
Source: Reuters, AFP