Malaysian authorities have announced that they are now investigating the possibility a missing Malaysian Airlines plane had turned back before disappearing, and are widening the search area accordingly.
As the search of the Malaysia Airlines flight continued into a second day on Sunday, Malaysian aviation authorities said it was "fearing the worst" and that radar displays indicated the plane could have turned around.
Al Jazeera's Scott Heidler, reporting from Kuala Lumpur, said the search would now be extended to the west coast of Malaysia.
"This raises a lot of unanswered questions," he said. "If the pilots had the wherewithal to turn around why did they not communicate with any of the towers? Still the officials say there wasn't a distress call."
No weather problems had been reported in the area before the plane dropped out of contact, and the pilots did not send a distress signal - something that has been highlighted by experts as unusual for a modern jetliner.
There was still no confirmed sighting of wreckage from the Boeing 777 in the seas between Malaysia and Vietnam where it vanished from screens early Saturday morning en route to Beijing from Kuala Lumpur.
The identities of four passengers on board the missing jetliner are being investigated over "airline security fears" as planes and ships from across Asia resumed the hunt for the plane that disappeared with 239 people on board.
Foreign Ministry officials in Rome and Vienna said names of two nationals listed as passengers matched passports reported stolen in Thailand. Malaysian Transport Minister Hishamuddin Hussein said authorities were also checking the identities of two other passengers.
Our correspondent said that two men travelling under stolen passports had booked their tickets via a Chinese website, and that their ticket numbers were consecutive - indicating they could have been travelling together.
Both men are believed to have booked onward tickets to Europe.
| Families of missing passengers await news in China
"All the four names are with me," said Hishamuddin, who is also defence minister. "I have indicated to our intelligence agencies and I have also spoken to international intelligence agencies for assistance."
He said help was also being sought from the US Federal Bureau of Investigation (FBI). However, an attack was only one of the possibilities being investigated.
"We are looking at all possibilities," he said. "We cannot jump the gun. Our focus now is to find the plane."
Vietnamese naval boats sent from the holiday island of Phu Quoc patrolled stretches of the Gulf of Thailand, searching for any wreckage, scouring the area where an oil slick was spotted by patrol jets just before nightfall on Saturday.
On Sunday, the search efforts were increased, with 22 aircraft and 40 ships from various authorities now scouring the original search area and the area to the west that could have been reached if the plane did turn around.
Li Jiaxiang, administrator of the Civil Aviation Administration of China, said some debris had been spotted, but it was unclear whether it came from the plane.
Vietnamese authorities said they had seen nothing close to two large oil slicks they saw Saturday and said to possibly be from the missing plane.
Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort.
Terrorism is always considered a possibility, but the sudden disappearance of Flight MH370 has given extra emphasis to speculation a bomb might have been on board.