The rationale used by the Reuters source:
"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 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.
Reuters goes on to say that the source spoke before reports indicating wreckage/debris had been spotted.