(AP) - Vietnamese authorities searching waters for the missing Boeing 777 jetliner spotted an object Sunday that they suspected was one of the plane's doors, as international intelligence agencies joined the investigation into two passengers who boarded the aircraft with stolen passports.

More than a day and half after Malaysia Airlines Flight MH370 went missing, no confirmed debris from the plane had been found, and the final minutes before it disappeared remained 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.

The state-run Thanh Nien newspaper cited Lt. Gen. Vo Van Tuan, deputy chief of staff of Vietnam's army, as saying searchers in a low-flying plane had spotted an object suspected of being a door from the missing jet. It was found in waters about 90 kilometers (56 miles) south of Tho Chu island, in the same area where oil slicks were spotted Saturday.

"From this object, hopefully (we) will find the missing plane," Tuan said. Thanh Nien said two ships from the maritime police were heading to the site.

The missing plane 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 the 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.

"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.

White House Deputy National Security Adviser Tony Blinken said the U.S. was looking into the stolen passports, but that investigators had reached no conclusions.

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 34 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.

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.

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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.