Questions mounted over possible security lapses and whether a bomb or hijacking could have brought down the Beijing-bound plane, after Interpol confirmed at least two passengers used stolen passports and said it was checking whether others aboard had used false identity documents.
Flight MH370 disappeared in the early hours of Saturday, about an hour into its flight from Kuala Lumpur, after climbing to a cruising altitude of 35,000 ft (10,670 metres).
The Civil Aviation Authority of Vietnam said on its website late on Sunday that a Vietnamese navy plane had spotted an object in the sea suspected of being part of the Boeing 777-200ER, but that it was too dark to be certain.
"We sent two boats to where the navy plane reported seeing that object but the boats couldn't find it," Admiral Ngo Van Phat told Reuters early on Monday. "We are sending more planes there this morning."
Shares in Malaysia Airlines fell as much as 18 percent to a record low on Monday morning.
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 said the failure to find any debris for two days, despite dozens of vessels and aircraft crisscrossing the sea below the flight path, indicated the plane may have broken up mid-flight.
"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.
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.
Still, 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.
The United States extensively reviewed imagery taken by American spy satellites for evidence of a mid-air explosion, but saw none at all, an authoritative U.S. government source said. The source described U.S. satellite coverage of the region as thorough.
Boeing declined to comment and referred to its brief earlier statement that said it was monitoring the situation.
More than 20 aircraft and 40 ships from seven nations are involved in the search. The U.S. Navy said its aircraft had located a previously reported oil slick south of Vietnam and east of Malaysia, but did not find any debris from the plane.
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.
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.
Malaysia's state news agency quoted Home Minister Ahmad Zahid Hamidi as saying the two passengers using the stolen European passports were of Asian appearance, and criticising border officials who let them through.
"I am still perturbed. Can't these immigration officials think? Italian and Austrian but with Asian faces," he was quoted as saying late on Sunday.
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