Radar signals show a Malaysia Airlines plane that has been missing since Saturday may have turned back, say officials.
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.
The Boeing 777-200 aircraft disappeared with 239 passengers and crew on board and is presumed to have crashed off the Vietnamese coast on Saturday.
There were no reports of bad weather and no sign of why the plane would have vanished from radar screens about an hour after it took off from Kuala Lumpur.
The BBC reports air and sea rescue teams have been searching an area of the South China Sea south of Vietnam for more than 24 hours.
But Malaysia's civil aviation chief, Azharuddin Abdul Rahman, told a media conference in Kuala Lumpur 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.
Twenty-two aircraft and 40 ships are now involved in the search, armed forces chief Zulkefli Zin said.
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.
Vietnamese navy ships which reached two oil slicks spotted earlier in the South China Sea found no signs of wreckage.
Malaysia's transport minister, Hishammuddin Hussein, initially said at least four names on the passenger list were "suspect". However, he later told the BBC there were in fact only two suspect names.
Reports suggest two of the passengers listed as travelling - an Italian and an Austrian - were not actually on the flight. They had both reportedly had their passports stolen in Thailand in recent years.
Mr Hussein said international agencies including the FBI had joined the investigation and all angles were being examined.
Two New Zealanders named
Malaysia Airlines has released the names of two New Zealanders aboard the flight.
The two passengers are Ximin Wang, 50, and Paul Weeks, 39.
New Zealand's Ministry of Foreign Affairs and Trade says it is in contact with the next of kin of the two families and is providing consular assistance.
The family of Mr Wang has left the country and do not intend to return for at least two months, a woman at Mr Wang's Auckland home said on Sunday morning.
Mr Wang is described in the electoral roll as a student.
Prime Minister John Key says he recognises the almost unbearable wait for the two New Zealand families and that while the families will be hoping for a miracle, they will also be preparing for the worst.
The passengers on the flight included 13 other nationalities, in addition to the New Zealanders: 153 Chinese citizens, 38 Malaysians, 7 Indonesians, 5 Indians, 6 Australians, 3 French, 4 Americans, 2 Ukrainians, 2 Canadians, 1 Russian, 1 Italian, 1 Taiwanese, 1 Dutch and 1 Austrian.
The Boeing, which was 11 years old and powered by Rolls-Royce Trent engines, took off at 12:40am on Saturday local time from Kuala Lumpur and was apparently flying in good weather when it went missing without a distress call.
Flight MH370 last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu.