International police agency Interpol confirmed at least two passports recorded in its database as lost or stolen were used by passengers on the flight, raising suspicions of foul play.
Nearly 48 hours after the last contact with Flight MH370, mystery still surrounded its fate. Malaysia's air force chief said yesterday the Beijing-bound airliner may have turned back from its scheduled route before it vanished from radar screens.
A source involved in the investigation said: "The fact we are unable to find any debris so far appears to indicate the aircraft is likely to have disintegrated at around 35,000ft."
If the plane had plunged intact from close to its cruising altitude, breaking up only on impact with the water, search teams would have expected to find a fairly concentrated pattern of debris.
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 causes.
Dozens of military and civilian vessels have been criss-crossing waters beneath the aircraft's flight path, but have found no confirmed trace of the lost plane, although oil slicks have been reported in the sea south of Vietnam and east of Malaysia.
A Vietnamese navy plane spotted an object in the sea suspected of being part of the plane, but it was too dark to be certain.
The Malaysian authorities said they were widening the search to cover vast swathes of sea around Malaysia and off Vietnam, and were investigating at least two passengers who were using false identity documents.
The passenger manifest issued by the airline included the names of two Europeans - Austrian Christian Kozel and Italian Luigi Maraldi - who, according to their foreign ministries, were not on the plane. Both had apparently had their passports stolen in Thailand during the past two years. The men falsely using their passports had purchased tickets together and were due to fly on to Europe from Beijing, meaning they did not have to apply for a Chinese visa and undergo further checks.
Interpol maintains a vast database of more than 40 million lost and stolen travel documents and has long urged member countries to make greater use of it to stop people crossing borders on false papers.
The global police organisation confirmed the two passports had both been added to the database after their theft in 2012 and 2013 respectively. But it said no country had consulted the database to check either of them since the time they were stolen.
Interpol secretary general Ronald Noble said: "It is of great concern any passenger was able to board an international flight using a stolen passport listed in Interpol's databases."
In a sign that Malaysia's airport controls may have been breached, Prime Minister Najib Razak said security procedures were being reviewed.
Malaysian Transport Minister Hishammuddin Hussein said authorities were also checking the identities of two other passengers.