THE disappearance of a Malaysian airliner about an hour into a flight to Beijing is an "unprecedented mystery", a civil aviation chief has said.
His comment came as a massive air and sea search 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 as questions mounted over possible security lapses and whether a bomb or hijacking attempt could have brought down the Boeing 777-200ER, which took off from the Malaysian capital Kuala Lumpur.
Azharuddin Abdul Rahman, the head of Malaysia's Civil Aviation Authority, said the area of the search would be widened.
A senior police official said people armed with explosives and carrying false identity papers had tried to fly out of Kuala Lumpur in the past and investigations were focused on two passengers who were on the missing plane with stolen passports.
The official said: "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. There have been two or three incidents, but I will not divulge the details."
Interpol has confirmed at least two passengers used stolen passports and said it was checking whether others aboard had used false identity documents.
Mr Azharuddin said a hijacking attempt could not be ruled out as investigators explore all theories for the loss of Malaysia Airlines Flight MH370.
He said: "Unfortunately we have not found anything that appears to be objects from the aircraft, let alone the aircraft. We have to find the aircraft. We have to find a piece of the aircraft if possible."
Mr Azharuddin also said the two men with stolen passports did not look like Asians but he did not elaborate. Airport CCTV footage showed they completed all security procedures, he said, adding: "We are looking at the possibility of a stolen passport syndicate."
A senior source involved in investigation 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 source 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."
Asked about the possibility of an explosion, the source said there was no evidence of foul play and the aircraft could have broken up due to mechanical causes.
The source said the closest parallels were the bomb explosions on board the PanAm aircraft over Lockerbie in 1988 and an Air India jetliner in 1985 when it was over the Atlantic Ocean.
The US has reviewed imagery taken by American spy satellites for evidence of a mid-air explosion after Saturday's flight disappeared but saw none.
Hopes of a breakthrough in the search for wreckage rose briefly yesterday 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 later the object had turned out to be a moss-covered cap of a cable reel.
Meanwhile, samples of oil taken from a slick in the South China Sea came from a ship, not the missing aircraft, it has been confirmed.