(Adds no comment from Boeing)
By Siva Govindasamy
KUALA LUMPUR, March 9 (Reuters) - Officials investigating the disappearance of a Malaysian airliner with 239 people on board are narrowing the focus of their inquiries on the possibility that it disintegrated mid-flight, a senior source said on Sunday.
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.
Malaysian authorities have said they are focused on finding the plane and have declined to comment when asked about the investigations.
However, the source said the closest parallels were the explosion on board an Air India jetliner in 1985 when it was over the Atlantic Ocean and the Lockerbie air disaster in 1988. Both planes were cruising at around 31,000 feet when bombs exploded on board.
Canadian and Indian police have long alleged the Air India bombing was conducted by Sikh extremists living in western Canada as revenge on India for the deadly 1984 assault on the Golden Temple in Amritsar, Sikhism's holiest shrine.
The bombing of Pan Am Flight 103 over the Scottish town of Lockerbie killed 259 passengers and crew and another 11 people on the ground. A Libyan intelligence officer was convicted for the attack.
International police agency Interpol has said at least two of the passengers on board the Malaysian plane, and possibly more, used passports listed as missing or stolen on its database.
"Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol's databases," Interpol Secretary General Ronald Noble said in a statement.
U.S. and European security officials have however maintained there is no proof yet of foul play and there could be other explanations for the use of stolen passports. (Editing by Raju Gopalakrishnan, Jeremy Laurence and Neil Fullick)