The disappearance of a Malaysian jetliner is an "unprecedented aviation mystery", a senior official said on Monday, the third day of a massive air and sea search to find any confirmed trace of the plane or the 239 people aboard.
The head of Malaysia's Civil Aviation Authority, Azharuddin Abdul Rahman, said a hijacking attempt could not be ruled out as investigators explore all theories for the loss of Malaysia Airlines flight MH370 en route to Beijing.
"Unfortunately we have not found anything that appears to be objects from the aircraft, let alone the aircraft," he told a news conference.
"As far as we are concerned, we have to find the aircraft, we have to find a piece of the aircraft if possible."
As dozens of ships and aircraft from seven countries scoured the seas around Malaysia and south of Vietnam, questions mounted over possible security lapses and whether a bomb or hijacking attempt could have brought down the Boeing 777-200ER airliner.
The passenger manifest issued by the airline included the names of two Europeans - Austrian Christian Kozel and Italian Luigi Maraldi - who were not on the plane.
Their passports had been stolen in Thailand during the past two years.
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An Interpol spokeswoman said a check of all documents used to board the plane had revealed more "suspect passports", which were being investigated.
"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.
Hopes for a breakthrough rose briefly when Vietnam scrambled helicopters to investigate a floating yellow object it was thought could have been a life raft.
But the country's Civil Aviation Authority said on its website that the object turned out to be a "moss-covered cap of a cable reel".
Flight MH370 disappeared from radar screens in the early hours of Saturday, about an hour into its flight from Kuala Lumpur, after climbing to a cruising altitude of 35,000ft.
A US Navy P-3 aircraft capable of covering 1,500 sq miles every hour was sweeping the northern part of the Strait of Malacca, on the other side of the Malaysian peninsula from where the last contact with MH370 was made.
"Our aircraft are able to clearly detect small debris in the water, but so far it has all been trash or wood," said US 7th Fleet spokesman Commander William Marks in an emailed statement.
No distress signal was sent from the lost plane, which experts said suggested a sudden catastrophic failure or explosion, but Malaysia's air force chief said radar tracking showed it may have turned back from its scheduled route before it disappeared.
A senior source involved in preliminary investigations in Malaysia told Reuters news agency the failure to quickly find any debris indicated the plane may have broken up mid-flight, which could disperse wreckage over a very wide area.
"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.