UPDATED 2pm: Kuala Lumpur, Malaysia: Investigators suspect Malaysia Airlines flight 370 may have disintegrated midair as search ships and planes scramble to confirm the sighting of possible wreckage of the aircraft's tail and interior door in the Gulf of Thailand.
But officials in Kuala Lumpur insisted on Monday the sighting of what might be wreckage was inconclusive.
"We have to wait to confirm," an official said.
The Malaysia Airlines Boeing 777 disappeared with 239 people, including six Australians, on board on Saturday.
The plane lost contact with ground controllers between Malaysia and Vietnam after leaving Kuala Lumpur for Beijing early on Saturday.
More than two days after flight MH370 went missing, the final minutes before it disappeared remain a mystery.
Airline executives and Malaysian aviation and defence officials are scheduled to brief the media at noon Malaysian time (3pm Sydney/Melbourne time).
The search area was expanded on on Sunday after Malaysian defence officers reviewed radar logs indicating the plane may have turned around in flight, which would indicate it was experiencing some difficulty.
But the pilots did not send a distress call.
Officials with knowledge of the investigation say the suspicion the plane disintegrated midair was partly fuelled by the inability to find a concentrated pattern of debris.
But investigators have not ruled out any possibility, including terrorism.
Failed to board
Four passengers on flight 370 failed to board after checking in their luggage, which raised further suspicion about the passengers after the plane disappeared.
But Malaysia’s Department of Civil Aviation chief Azaharuddin Abdul Rahman told Fairfax Media the passengers' luggage was offloaded from the plane before it left Kuala Lumpur airport in the early hours of Saturday morning.
Is this the door from the missing Malaysian Airlines plane?
He said the luggage was screened and found not to contain anything suspicious and was then returned to the passengers in the terminal.
"We followed standard operating procedures to remove the baggage of those who didn’t turn up," he said.
"There was nothing suspicious [about those passengers]," he said.
The identities of the four passengers have not been made public.
A Vietnamese plane reportedly sighted the debris in the waters of the Gulf of Thailand.
The Vietnamese Information Ministry made the announcement on its website on Sunday, saying the objects appeared to be a fragment of an aircraft's tail and an interior door.
The objects were located about 90 kilometres south of the island of Tho Chu, in the same area where the plane could have gone down if it did not alter its route, a possibility that is currently being investigated. It is in the same area where oil slicks were spotted on Saturday.
The discovery came shortly before nightfall, when air operations were cancelled until Monday morning.
The report was transmitted to boats in the area that are participating in search and rescue operations so they can recover the objects and determine precisely what they are and where they came from.
China, the United States, the Philippines, Indonesia, Malaysia, Singapore, Thailand and Vietnam, shortly to be joined by Australia, are co-operating in the search for the Boeing 777-200.
Vietnamese authorities searching waters for the missing jet spotted an object on Sunday that they suspected was one of the plane's doors, as international intelligence agencies joined the investigation into two passengers who boarded the aircraft with stolen passports.
The state-run Thanh Nien newspaper earlier cited Lieutenant General Vo Van Tuan, deputy chief of staff of Vietnam's army, as saying searchers in a low-flying plane had spotted an object suspected of being a door from the missing jet. It is unclear at this stage if this is the same door mentioned on the Vietnamese Information Ministry's website.
"From this object, hopefully [we] will find the missing plane," General Tuan said. Thanh Nien said two ships from the maritime police were heading to the site.
Officials investigating the disappearance of the airliner are narrowing the focus of their inquiries on the possibility that it disintegrated mid-flight, a senior source said on Sunday.
Search teams have not been able to make any confirmed discovery of wreckage in seas beneath the plane's flight path almost 48 hours after it took off.
"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, who is involved in the preliminary investigations in Malaysia.
If the plane had plunged intact from such a height, breaking up only on impact with the water, search teams would have expected to find a fairly concentrated pattern of debris, said the source, speaking on condition of anonymity because he was not authorised to speak publicly on the investigation.
The missing plane apparently fell from the sky at cruising altitude in fine weather, and the pilots were either unable or had no time to send a distress signal - unusual circumstances under which a modern jetliner operated by a professional airline would crash.
Malaysia's air force chief, Rodzali Daud, said radar indicated that the plane may have turned back, but did not give further details on which direction it went or how far it might have veered off course.
"We are trying to make sense of this," Mr Daud said at a news conference. "The military radar indicated that the aircraft may have made a turn back, and in some parts this was corroborated by civilian radar."
Malaysia Airlines chief executive Ahmad Jauhari Yahya said pilots were supposed to inform the airline and traffic control authorities if the plane made a U-turn.
"From what we have, there was no such distress signal or distress call per se, so we are equally puzzled," he said.
Authorities were checking on the identities of the two passengers who boarded the plane with stolen passports. On Saturday, the foreign ministries in Italy and Austria said the names of two citizens listed on the flight's manifest matched the names on two passports reported stolen in Thailand.
"I can confirm that we have the visuals of these two people on CCTV," acting Malaysian Transport Minister Hishammuddin Hussein said at a news conference late on Sunday, adding that the footage was being examined.
"We have intelligence agencies, both local and international, on board."
Mr Hishammuddin declined to give further details, saying it may jeopardise the investigation.
"Our focus now is to find the aircraft," he said, adding that finding the plane would make it easier for authorities to investigate any possible foul play.
Interpol confirmed that at least two stolen passports used by passengers on the plane were registered in its databases. It said no one had checked the databases, but added that most airlines and countries do not usually check for stolen passports.
Mr Hishammuddin said only two passengers had used stolen passports, and that earlier reports that the identities of two others were under investigation were not true.
White House Deputy National Security Adviser Tony Blinken said the US was looking into the stolen passports issue, but that investigators had reached no conclusions.
In addition to the plane's sudden disappearance, which experts say is consistent with a possible onboard explosion, the stolen passports have strengthened concerns about terrorism as a possible cause. Al-qaeda militants have used similar tactics to try to disguise their identities.
Still, other possible causes would seem just as likely at this stage, including a catastrophic failure of the plane's engines, extreme turbulence, or pilot error or even suicide. Establishing what happened with any certainty will need data from flight recorders and a detailed examination of any debris, something that will take months if not years.
European authorities on Saturday confirmed the names and nationalities of the two stolen passports: one was an Italian-issued document bearing the name Luigi Maraldi, the other Austrian under the name Christian Kozel. Police in Thailand said Mr Maraldi's passport was stolen on the island of Phuket last July.
A telephone operator on a China-based KLM hotline on Sunday confirmed that "Maraldi" and "Kozel" were both booked to leave Beijing on a KLM flight to Amsterdam on March 8. Mr Maraldi was then to fly to Copenhagen, Denmark, on KLM on March 8, and Mr Kozel to Frankfurt, Germany, on March 8.
She said that, since the pair booked the tickets through China Southern Airlines, she had no information on where they bought them.
Having onward reservations to Europe from Beijing would have meant the pair, as holders of EU passports, would not have needed visas for China.
Meanwhile, the multinational search for the missing plane was continuing. A total of 34 aircraft and 40 ships have been deployed to the area by Malaysia, Thailand, Australia, Singapore, Indonesia, China and the United States, in addition to Vietnam's fleet.
Vietnamese air force jets spotted two large oil slicks on Saturday, but it was unclear whether they were linked to the missing plane.
Two-thirds of the jet's passengers were Chinese. The rest were from elsewhere in Asia, North America and Europe.
After more than 30 hours without contact with the aircraft, Malaysia Airlines told family members they should "prepare themselves for the worst", Hugh Dunleavy, the commercial director for the airline, told reporters.
Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over many square kilometres. If the plane enters the water before breaking up, there can be relatively little debris.
A team of American experts was en route to Asia to be ready to assist in the investigation into the crash. The team includes accident investigators from the National Transportation Safety Board, as well as technical experts from the Federal Aviation Administration and Boeing, the safety board said in a statement.
Malaysia Airlines has a good safety record, as does the 777, which had not had a fatal crash in its 19-year history until an Asiana Airlines plane crashed last July in San Francisco, killing three passengers, all Chinese teenagers.
Meanwhile, the US team confirmed the floating object spotted by a Singaporean aircraft on Sunday is not linked to the Malaysia Airlines plane.
EFE, AP, Reuters
Chris Brummitt reported from Hanoi, Vietnam. Associated Press writers Rod McGuirk in Canberra, Australia; Didi Tang, Gillian Wong and Louise Watt in Beijing; Joan Lowy in Washington; and Scott Mayerowitz in New York contributed this report.