Military radar indicates that the missing Boeing 777 jet may have turned back before vanishing, Malaysia's air force chief said as authorities were investigating up to four passengers with suspicious identifications.
The revelations add to the uncertainties surrounding the final minutes of flight MH370, which was carrying 239 people when it lost contact with ground controllers somewhere between Malaysia and Vietnam after leaving Kuala Lumpur early Saturday morning for Beijing.
A massive international sea search has so far turned up no trace of the plane, which lost contact with the ground when the weather was fine, the plane was already cruising and the pilots didn't send a distress signal - unusual circumstance for a modern jetliner operated by a professional airline to crash.
Vietnamese air force jets spotted two large oil slicks Saturday, but it was unclear if they were linked to the missing plane, and no debris was found nearby.
Air force chief Rodzali Daud didn't say which direction the plane veered when it apparently went off course, or how long it flew in that direction.
"We are trying to make sense of this," he told a media 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 does 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 suspect identities of at least two passengers who appear to have boarded 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.
Malaysian Transport Minister Hishammuddin Hussein said that authorities were looking at two more possible cases of suspicious identities.
He said Malaysian intelligence agencies were in contact with their international counterparts, including the FBI. He gave no more details.
"All the four names are with me and have been given to our intelligence agencies," he said. "We are looking at all possibilities."
The stolen passports, and the sudden disappearance of the plane that experts say is consistent with a possible onboard explosion, strengthened existing concerns about terrorism as a possible cause for the disappearance. Al-Qaida militants have used similar tactics to try and disguise their identities.
Despite that, other possible causes would seem just as likely at this stage, including a catastrophic failure of the engines or the plane itself, extreme turbulence and pilot error or even suicide.
Establishing a cause 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.
A telephone operator on a China-based KLM hotline on Sunday confirmed "Maraldi" and "Kozel" were both booked to leave Beijing on a KLM flight to Amsterdam on March 8. Maraldi was then to fly to Copenhagen on KLM on March 8, and Kozel to Frankfurt on March 8.
She said the pair booked the tickets through China Southern Airlines so she had no information on where they bought them.
Having an onward reservation from Beijing would have meant the pair, as holders of EU passports, would not have needed a visa for China. Beyond that, it was unclear whether this had any possible implications for the investigation.
A total of 22 aircraft and 40 ships have been deployed to the area by Malaysia, Thailand, Singapore, Indonesia, China and the United States, not counting Vietnam's fleet.
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 kilometers.
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 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.
Jason Middleton, the head of the Sydney-based University of New South Wales' School of Aviation, said terrorism or some other form of foul play seemed a likely explanation.
"You're looking at some highly unexpected thing, and the only ones people can think of are basically foul play, being either a bomb or some immediate incapacitating of the pilots by someone doing the wrong thing and that might lead to an airplane going straight into the ocean," Middleton said.
"With two stolen passports (on board), you'd have to suspect that that's one of the likely options."
But Clive Williams, a counter-terrorism expert at Australia's Macquarie University and a former military intelligence officer, said he doubted the two stolen passports aboard the flight were related to the disaster.
The latest Interpol data showed there were 39 million lost or stolen passports reported as of December 2013.
"Any flight of that size in Asia would be carrying a couple of people with false passports," he said.
"When you think about the number of passports that have been stolen or gone missing around the world ... it could be related, but it's probably not.
FAMILIES CLING TO HOPE
The family of a former Christchurch man who was on board the missing Malaysia Airlines jet are today clinging to hope at his grandparents' house while bracing themselves for the worst.
Paul Weeks was one of two New Zealanders named on a passenger list for Malaysia Airlines flight MH370. The other Kiwi was named as Ximin Wang, 50.
Weeks left Christchurch for Perth in 2011 with his wife, Danica, and two young children in search of a better life after the earthquakes.
The 38-year-old was en route to Mongolia for his new job with Transwest Mongolia.
His emotional wife, Danica, said on Sunday she was desperately waiting for news of him.
"I can't give up hope. I would love him to walk through that door, hold him one more time ... I see him everywhere in the house," she told the Nine Network.
"It's so hard, so hard. I mean we are praying for a miracle."
The couple have a three-year-old son, Lincoln, and a 10-month-old son, Jack.
Prime Minister John Key today expressed his sadness over the aircraft's disappearance.
"This will be an almost unbearable wait for the families. Not knowing what has happened to the flight and their loved ones is an awful ordeal. While they will be hoping for a miracle, they will also be preparing themselves for the worst," he said.
"There is nothing I can say to alleviate their pain, but they should know we are hoping and praying with them.
"The thoughts of New Zealanders everywhere are with them at this most distressing time."
Paul Weeks' mother, who followed him to Perth and "lives about two doors down on the same street'' flew to Christchurch this morning on a pre-booked flight, Sara Weeks said, to attend her 40th birthday next weekend.
"Everyone other than my grandparents moved over there. My mother, my youngest brother and my oldest daughter all flew over this morning. We're with my grandparents. We're just basically sitting here watching the news," she said.
"We're obviously very upset. But you kind of cling to that little bit of hope when you don't know."
Sara Weeks said she had not seen her brother since he moved to Perth, but they kept in regular contact on Skype.
"He had just taken on a new role. That's why he was heading to Mongolia. He was going to be based there for a month on [at a time]. It was a really good job and he was going to be paid very well," she said.
"He was excited and looking forward to getting started. It was going to set them up. When [Danica] kissed him goodbye she was hoping he would be back in a month."
Sara Weeks said her brother was a "lovely man", "lots of fun" and "very family-oriented".
"He has some very good friends that were in the army with him and from when he was growing up," she said.
Weeks attended Aranui High School before studying at the University of Canterbury. He previously worked for the New Zealand Army.
SEEKING A BETTER LIFE
Weeks spoke to the Press in 2012 saying he would have preferred to stay in New Zealand but the odds were stacked against them.
"Career-wise it is far better in Australia. There is a mining boom here. I sent out 100 job applications before moving and within a week had three job offers. It is chalk and cheese with what is happening in New Zealand," Weeks told The Press in 2012.
Weeks told the Press he doubled his salary and said with the effect of the exchange rate, probably tripled it.
"I consider our move to Australia to be one of necessity, rather than by choice, as we were content starting a family in Christchurch and enjoying the Canterbury outdoors. However, there is only so much an average family can take before one abandons the nest."
He blamed recessionary pressure, high food prices and the continuing rumbling of after shocks in Christchurch for forcing his decision to leave.
"After being in Perth for the past six months I can honestly say that the effort required to flourish in Australia is significantly lower than that of New Zealand."
"The cost of living is probably cheaper to some extent and cars are cheaper. The negatives are being away from family but it is pretty good here and you're never short of money."
The Ministry of Foreign Affairs and Trade said it was in contact with their next of kin and providing consular assistance to the families.
- AP, Fairfax, Agencies