Investigators in Malaysia say the lack of debris from the vanished Kuala Lumpur to Beijing flight could indicate it "disintegrated" in midair.
"The fact that we are unable to find any debris so far appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet," said a source who is involved in the investigations in Malaysia.
Read more of the Herald's Flight 370 coverage today:
Missing plane: Security beefed up for Malaysia flights
Missing plane: From an acclaimed calligrapher to a young man off to begin a new career, all passengers had a story to tell
Missing plane: Families flown to Malaysia
Asked about the possibility of an explosion, such as a bomb, the source said there was no evidence yet of foul play and the aircraft could have broken up because of mechanical problems.
But David Learmount, operations and safety editor for
, said he would be very surprised if the authorities knew for sure that the plane "disintegrated" in midair.
He added that it was not unusual to fail to find debris immediately after a crash, pointing out that it took time to find the evidence and uncover the facts.
"We just have to accept that, for the moment, we do not know what has happened," he said.
"It's also worth remembering that it took two days for any wreckage from the Air France crash in June 2009 to be found - and two years for the full operation to be completed."
Earlier, Vietnamese authorities said a military plane had spotted debris suspected to be part of the missing airliner. But less than an hour later, Vietnam said the objects had turned out to be nothing to do with the plane.
Datuk Azharuddin Abdul Rahman, director-general of Malaysia's Department of Civil Aviation, said the debris was not from the plane.
He added that the search radius had been expanded from 20 nautical miles to 50 nautical miles.
"At present, we have 34 aircraft and 40 ships currently combing the search and rescue areas."
Amid speculation that foul play may have been behind the plane's demise, Rahman said investigators were looking at CCTV footage of the two passengers who boarded the plane with stolen passports.
"The video of the two passengers is now being looked at," he said in Kuala Lumpur. "We are looking at all angles and all possibilities."
Rahman declined to provide further details of what examination of the CCTV footage had so far shown.
But he added: "On the possibility of hijack, we are not ruling out any possibility. However, it is important to state that our main concern is to focus our effort to find the missing aircraft."
Rahman said the main priority of investigators was to locate any wreckage from the plane. Officials would then be able to examine the "black-box" flight recorder to ascertain further clues. Crucially, no signal has been received from the plane's emergency locator transmitter.
Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over a large area. If the plane enters the water before breaking up, there can be relatively little debris.
Andrew Charlton, an aviation consultant at Geneva-based Aviation Advocacy, said there was much that did not add up.
"The 777 is a very reliable aircraft, Malaysia is a very good airline and it had cleared takeoff and landing. For the aircraft not to have been able to talk to the ground is really most alarming and concerning," he said. "It just disappeared off the face of the map. When this happens it's catastrophic and instantaneous, and it's very difficult not to assume an explosion was involved at that point."
If all on the Malaysia Airlines flight died, then it will be the deadliest aircraft accident since November 12, 2001, when American Airlines Flight 587 crashed in New York, killing all 260 people on board and five on the ground.
Pilot tells of last contact with jet
A pilot who was flying near the missing Malaysia Airlines flight has come forward, saying he made radio contact with the missing jet moments after it dropped off the radar.
The unnamed pilot told Malaysia's New Straits Times newspaper that a Vietnamese control tower had asked him to contact the plane.
Using his aircraft's emergency frequency, he tried to get in contact with the plane after authorities were unable to get in touch.
"We managed to establish contact with MH370 just after 1.30am and asked them if they have transferred into Vietnamese airspace," he said.
"The voice on the other side could have been either Captain Zaharie or Fariq, but I was sure it was the co-pilot.
"There were a lot of interference ... static ... but I heard mumbling from the other end.
"That was the last time we heard from them, as we lost the connection."
Despite losing contact, which often happens on the emergency frequency, the pilot did not suspect a problem until the plane failed to arrive in Beijing.
"If the plane was in trouble, we would have heard the pilot making the mayday distress call. But I am sure that, like me, no one else up there heard it," he said. "Following the silence, a repeat request was made by the Vietnamese authorities to try establishing contact with them."
- additional reporting Independent, AP, AFP