Radar signals show that a Malaysia Airlines plane missing for more than 24 hours may have turned back, Malaysian officials have said.
Rescue teams looking for the plane have now widened their search area.
Investigators are also checking CCTV footage of two passengers who are believed to have boarded the plane using stolen passports.
Flight MH370 from Kuala Lumpur to Beijing disappeared south of Vietnam with 239 people on board.
Air and sea rescue teams have been searching an area of the South China Sea south of Vietnam for more than 30 hours, but there have been no definite sightings of wreckage.
Late on Sunday, the Vietnamese authorities said a navy aircraft had spotted "an object" suspected of belonging to the missing plane, but officials said it was too dark to be certain.
The object is thought to be near a potential oil slick that was spotted on Saturday, but again officials have urged that this may be nothing to do with flight MH370.
Malaysia's civil aviation chief, Azharuddin Abdul Rahman, told a press conference in Kuala Lumpur that the search area had been expanded to include the west coast of Malaysia.
Five passengers booked on the flight did not board, he added. Their luggage was consequently removed.
There are now 40 ships and 34 aircraft from nine different nationalities taking part in the search. But no signal has been received from the plane's emergency locator transmitter, Malaysian aviation authorities say.
Air force chief Rodzali Daud said the investigation was now focusing on a recording of radar signals that showed there was a "possibility" the aircraft had turned back from its flight path.
The BBC has confirmed that a man falsely using an Italian passport and a man falsely using an Austrian passport purchased tickets at the same time, and were both booked on the same onward flight from Beijing to Europe on Saturday.
Both had purchased their tickets from China Southern Airlines, which shared the flight with Malaysia Airlines, and they had consecutive ticket numbers.
The real owners had their passports stolen in Thailand in recent years.
The international police agency Interpol confirmed that at least two passports recorded as lost or stolen in its database were used by passengers on the flight - and that no checks of its database had been made for either passport between the time they were stolen and the departure of the flight.
"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 databases," the agency's Secretary General Ronald Noble said in a statement.
He expressed frustration that few of Interpol's 190 member countries "systematically" search the database.
Malaysia's Transport Minister Hishammuddin Hussein said international agencies including the FBI had joined the investigation and all angles were being examined.
"Our own intelligence have been activated and, of course, the counterterrorism units... from all the relevant countries have been informed," he said.
"The main thing here for me and for the families concerned is that we find the aircraft."
The passengers on the flight were of 14 different nationalities. Two-thirds were from China, while others were from elsewhere in Asia, North America and Europe.
The plane vanished from radar south of Vietnam at 17:30 GMT Friday (01:30 local time Saturday).
Malaysia Airlines had previously said it last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu.
Distraught relatives and loved ones of those on board are being given assistance at both the arrival and departure airports.
Many have expressed anger at the lack of information.
"I can't understand the airline company. They should have contacted the families first thing," a middle-aged woman told AFP news agency at Beijing airport, after finding out her brother-in-law was on the flight.
Texas firm Freescale Semiconductor says 20 of its Malaysian and Chinese employees were on the flight, according to a statement on its website.
Malaysia's national carrier is one of Asia's largest, flying nearly 37,000 passengers daily to some 80 destinations worldwide.
Correspondents say the route between Kuala Lumpur and Beijing has become more and more popular as Malaysia and China increase trade.