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Military radar suggests the missing Boeing 777 jet may have turned back before disappearing over the South China Sea, Malaysia's air force chief said Sunday.

Rodzali Daud told a press conference that "there is a possible indication that the aircraft made a turnback," adding that authorities were "trying to make sense of that."

Investigators were also checking surveillance footage of two passengers they believe boarded the Malaysia Airlines jet using stolen passports. Two names on the passenger manifest of the plane matched passports reported stolen in Thailand, one belonging to an Italian and the other to an Austrian. The passports' legitimate owners were not on the aircraft.

U.S. officials told NBC News on Saturday they were investigating terrorism concerns after the red-eye flight from Kuala Lumpur, Malaysia, to Beijing vanished in relatively clear weather, without sending a distress signal, at what analysts said would have been cruising altitude.

“We are aware of the reporting on the two stolen passports,�” one senior U.S. official said. “We have not determined a nexus to terrorism yet, although it’s still very early, and that’s by no means definitive.”

Officials note that there were other criminal reasons, such as drug smuggling, that stolen passports might be used to board a plane.

The Italian on the passenger list whose passport had been stolen was Luigi Maraldi, 37. His father, Walter Maraldi, told NBC News on Saturday that Luigi was vacationing in Thailand and had called to check in.

“We didn’t know about the accident,” the father said from Cesena, Italy. “Thank God he heard about it before us.”

Walter Maraldi said his son had his passport stolen a year ago in Thailand.

In Austria, the foreign ministry confirmed to NBC News that police had made contact with a citizen who was also on the passenger list, and who reported his passport stolen two years ago.

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“We believe that the name and passport were used by an unidentified person to board the plane,” a spokesman for the ministry said.

It is unusual, but not unheard of, for one person to board a plane with a stolen passport. It is very rare for two people with stolen passports to board the same plane, terrorism analysts say.

International police body Interpol maintains a database of more than 39 million travel documents reported lost or stolen by 166 countries, and says on its website that this enables police, immigration or border control officers to check the validity of a suspect document within seconds.

"As far as we're aware, every one of the people onboard that aircraft had a visa to go to China. Which means those passports were in the possession of the Chinese Embassy before those visas were issued," Malaysia Airlines' director of commercial operations Hugh Dunleavy said.

Malaysia's transport minister Hishammuddin Hussein said that at least four names on the passenger list were under investigation, but that authorities would also focus on the entire passenger manifest.

He said help was also being sought from the FBI.

"We are looking at all possibilities," he added. "We cannot jump the gun. Our focus now is to find the plane."

A Malaysian official with knowledge of the investigation told Reuters the passengers being checked had bought their tickets through China Southern Airlines, which was code-sharing the flight with Malaysia Airlines.

According to Malaysia's civil aviation chief Azharuddin Abdul Rahman, five passengers booked on the flight did not board, and their luggage was removed.

Search operations continued Sunday for the missing jet, but officials said no sightings of the wreckage had been reported after an all-night search. Authorities confirmed an oil slick had been spotted by in the water off Vietnam.

The U.S. National Transportation Safety Board said it had dispatched a team to help investigate the crash. The NTSB team is accompanied by technical advisers from the Federal Aviation Administration and Boeing, which made the 777-200ER jet.

The aircraft was carrying 227 passengers and 12 crew. Most of the passengers were Chinese. Three were Americans — one adult and two children, according to the passenger manifest.

In Beijing, anguished families gathered at the airport and were taken to a hotel to wait for what little information there was.

The airline asked for prayers from the world.

The investigation is expected to be lengthy, partly because authorities would have to find wreckage and perform forensic tests. It took more than a year to rule out terrorism after the 1996 crash of TWA Flight 800 off Long Island.

When Air France Flight 447 went down in the Atlantic Ocean in June 2009, bodies and some parts of the plane were found within two weeks, but it took two years to find the main wreckage.

While flight data recorders and cockpit voice recorders, the so-called black boxes, can emit signals from underwater, it can be extremely difficult to find planes that disappear over the sea.

The last fatal crash for Malaysia Airlines was in 1995, when 34 people were killed near the city of Tawau. In 1977, a domestic Malaysia Airlines flight was hijacked and crashed, killing 100 people.

Andy Eckardt, Claudio Lavanga, Erin McClam, Jason Cumming, F. Brinley Bruton, Becky Bratu and Michele Neubert of NBC News contributed to this report. Reuters and The Associated Press also contributed.