Malaysia Airlines flight MH370 vanished after climbing to a cruising altitude of 35,000 feet between Kuala Lumpur and Beijing in the early hours of Saturday. Search teams have not been able to make any confirmed discovery of wreckage in seas beneath the plane's flight path almost 48 hours after it took off.
"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 the source, who is involved in the preliminary investigations in Malaysia.
If the plane had plunged intact from such a height, breaking up only on impact with the water, search teams would have expected to find a fairly concentrated pattern of debris, said the source, speaking on condition of anonymity because he was not authorised to speak publicly on the investigation.
The source was speaking shortly before Vietnamese authorities said a military plane had spotted an object at sea suspected to be part of the missing airliner.
Asked about the possibility of an explosion, such as a bomb, the source said there was no evidence yet of foul play and that the aircraft could have broken up due to mechanical issues.
Boeing, the maker of the 777-200ER, declined to comment and referred to its earlier statement, which said it was monitoring the situation.