* No confirmed wreckage found, "life raft" report false alarm
* Officials say possibility of hijacking cannot be ruled out
* Questions over security after at least 2 stolen passports used
* Jet may have disintegrated in mid-air - investigation source
* 227 passengers and 12 crew presumed dead aboard lost flight
T he disappearance of a Malaysian jetliner is an "unprecedented mystery", the country's civil aviation chief said on Monday, as a massive air and sea search now in its third day failed to find any trace of the plane or 239 people on board.
Dozens of ships and aircraft from 10 countries scoured the seas around Malaysia and south of Vietnam, and questions mounted over possible security lapses and whether a bomb or hijacking attempt could have brought down the Boeing 777-200ER airliner flying from Kuala Lumpur to Beijing.
A senior police official told Reuters that people armed with explosives and carrying false identity papers had tried to fly out of Kuala Lumpur in the past, and that current investigations were focused on two passengers who were on the missing plane with stolen passports.
"We have stopped men with false or stolen passports and carrying explosives, who have tried to get past KLIA (airport) security and get on to a plane," he said. "There have been two or three incidents, but I will not divulge the details."
Interpol confirmed on Sunday at least two passengers used stolen passports and said it was checking whether others aboard had used false identity documents.
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.
"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."
A senior source involved in preliminary investigations in Malaysia said the failure to 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.
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 bomb explosions on board an Air India jetliner in 1985 when it was over the Atlantic Ocean and a Pan Am aircraft over the Scottish town of Lockerbie in 1988. Both planes were cruising at around 31,000 feet at the time.
The United States extensively reviewed imagery taken by American spy satellites for evidence of a mid-air explosion, but saw none, a U.S. government source said. The source described U.S. satellite coverage of the region as thorough.
MOSS-COVERED CABLE REEL
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 35,000 ft (10,670 metres).
Underlining the lack of hard information about the plane's fate, a U.S. 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.
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.
The Boeing 777 has one of the best safety records of any commercial aircraft in service. Its only previous fatal crash came on July 6 last year when Asiana Airlines flight 214 struck a seawall on landing in San Francisco, killing three people.
About two-thirds of the 227 passengers and 12 crew now presumed to have died aboard the plane were Chinese. The airline said other nationalities included 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans.
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.
A European diplomat in Kuala Lumpur cautioned that the Malaysian capital was an Asian hub for illegal migrants, many of whom used false documents and complex routes including via Beijing or West Africa to reach a final destination in Europe.
"You shouldn't automatically think that the fact there were two people on the plane with false passports had anything to do with the disappearance of the plane," the diplomat said.
"The more you know about the role of Kuala Lumpur in this chain, the more doubtful you are of the chances of a linkage."