KUALA LUMPUR, Malaysia (AP) -- Military radar indicates that the missing Boeing 777 jet may have turned back before vanishing, Malaysia's air force chief said Sunday 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.
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; pilot error or even suicide were all possible. Establishing a cause with any certainty will need data from flight recorders and a detailed examination of any debris, something that will 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 (miles). 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. He said latest Interpol data showed there were 39 million lost or stolen passports reported as of Dec. 13, 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."
Brummitt reported from Hanoi, Vietnam.
Associated Press journalist Rod McGuirk in Canberra; Didi Tang, Gillian Wong and Louise Watt in Beijing; Joan Lowy in Washington; and Scott Mayerowitz in New York contributed this report.