KUALA LUMPUR, Malaysia (AP) — International intelligence agencies joined the investigation Sunday into the two passengers who boarded the missing Boeing 777 jetliner with stolen passports, as Malaysian authorities revealed that radar images showed the plane may have turned back before vanishing.

More than a day and half after Malaysia Airlines Flight MH370 went missing, no debris from the plane has been found, and the final minutes before it disappeared remain a mystery. The plane, which was carrying 239 people, 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. The aircraft apparently fell from the sky at cruising altitude in fine weather, and the pilots were either unable or had no time to send a distress signal — unusual circumstances under which a modern jetliner operated by a professional airline would crash.

Malaysia's air force chief, Rodzali Daud, said radar indicated that the plane may have turned back, but did not give further details on which direction it went or how far it veered off course.

"We are trying to make sense of this," Daud said at a news 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 are 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 identities of two passengers who boarded the plane 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.

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"I can confirm that we have the visuals of these two people on CCTV," Malaysian Transport Minister Hishammuddin Hussein said at a news conference late Sunday, adding that the footage was being examined. "We have intelligence agencies, both local and international, on board."

Hishammuddin declined to give further details, saying it may jeopardize the investigation.

"Our focus now is to find the aircraft," he said, adding that finding the plane would make it easier for authorities to investigate any possible foul play.

Interpol confirmed that at least two stolen passports used by passengers on the plane were registered in its databases. It said no one had checked the databases, but added that most airlines and countries do not usually check for stolen passports.

Hishammuddin said only two passengers had used stolen passports, and that earlier reports that the identities of two others were under investigation were not true.

In addition to the plane's sudden disappearance, which experts say is consistent with a possible onboard explosion, the stolen passports have strengthened concerns about terrorism as a possible cause. Al-Qaida militants have used similar tactics to try and disguise their identities.

Still, other possible causes would seem just as likely at this stage, including a catastrophic failure of the plane's engines, extreme turbulence, or pilot error or even suicide. Establishing what happened 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. Police in Thailand said Maraldi's passport was stolen on the island of Phuket last July.

A telephone operator on a China-based KLM hotline on Sunday confirmed that "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, Denmark, on KLM on March 8, and Kozel to Frankfurt, Germany, on March 8.

She said since the pair booked the tickets through China Southern Airlines, she had no information on where they bought them.

Having onward reservations to Europe from Beijing would have meant the pair, as holders of EU passports, would not have needed visas for China.

Meanwhile, the multinational search for the missing plane was continuing. A total of 22 aircraft and 40 ships have been deployed to the area by Malaysia, Thailand, Australia, Singapore, Indonesia, China and the United States, in addition to Vietnam's fleet.

Vietnamese air force jets spotted two large oil slicks Saturday, but it was unclear whether they were linked to the missing plane, and no debris was found nearby.

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 the 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, you'd have to suspect that that's one of the likely options."

But Clive Williams, a counterterrorism expert at Australia's Macquarie University and a former military intelligence officer, said he doubted the stolen passports were related to the disaster. As of December, Interpol's databases contained 39 million lost or stolen passports.

"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."

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Brummitt reported from Hanoi, Vietnam.

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Associated Press writers Rod McGuirk in Canberra, Australia; Didi Tang, Gillian Wong and Louise Watt in Beijing; Joan Lowy in Washington; and Scott Mayerowitz in New York contributed this report.