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SEPANG, Malaysia – As military aircraft and a flotilla of ships from a half-dozen nations combed the waters south of Vietnam on Sunday for signs of a jet with 239 people onboard that vanished a day earlier, the authorities here deflected troubling questions about two flight passengers who had used passports listed in an international database as lost or stolen.

The secretary general of Interpol, Ronald K. Noble, said Sunday that no checks had been conducted by the authorities in Malaysia or any other country about the two passports before the plane, a Boeing 777-200, left on flight MH370, which disappeared Saturday en route to Beijing from Kuala Lumpur.

In a statement, Noble warned that “only a handful of countries” around the world routinely made such checks.

Security and aviation experts continued to offer starkly disparate theories on Sunday about why flight MH370 disappeared, a measure both of how little is known about its fate and of how little information the Malaysian authorities are revealing. In a series of briefings, Malaysian officials have refused to answer any questions relating to what they described as “security matters.”

The overnight flight left Kuala Lumpur International Airport for Beijing early Saturday and disappeared from radar about one hour after takeoff. No distress signal was sent, officials have said.

Azharuddin Abdul Rahman, the director general of Malaysia’s Department of Civil Aviation, said samples from an oil slick discovered Saturday were being tested.

“We are doing the verification of whether the oil comes from the aircraft or not,” he said.

Vietnamese news media reported that a yellow object that rescuers had believed might be part of the aircraft turned out to be a coral reef.

The plane was flying in apparently calm weather. Speculation on the reasons for its disappearance ranged from a rare, catastrophic mechanical failure to something more sinister.

More details emerged Sunday about the two passengers who used the fraudulently obtained passports. Both men bought one-way tickets issued last week at the same travel agency in a shopping mall in the Thailand beach resort of Pattaya, according to electronic booking records. A woman who answered the phone at the travel agency said she was “too busy to talk.”

It is unclear how the men traveled south to Malaysia to board the flight Saturday. In Beijing, each man was to continue to separate European cities, according to the electronic records. As transit passengers they would not have been required to obtain Chinese visas.

Security experts in Asia said the use of false travel documents is a persistent problem in the region but differed on the significance of the stolen passports to the investigation.

Xu Ke, a lecturer at the Zhejiang Police College in eastern China who studies aviation safety and hijackings and has advised the Chinese authorities, said the two men may have been illegal migrants.

“There are many cases of falsified and counterfeit passports and visas for illegal migration that our public security comes across, even several cases every day,” he said.

But Steve Vickers, the chief executive of a Hong Kong-based security consulting company that specializes in risk mitigation and corporate intelligence in Asia, said the presence of at least two travelers on stolen passports aboard a single jet is rare and a potential clue.

“It is fairly unusual to have more than one person flying on a flight with a stolen passport,” said Vickers, who publicly said a month ago that stolen airport passes and other identity documents in Asia merited a crackdown. “The future of this investigation lies in who really checked in and what they looked like,” he added.

Azharuddin, the Malaysian civil aviation chief, said investigators were reviewing video footage of the passengers.

“There are only two passengers on record that flew on this aircraft that had false passports,” he said. “And we have the CCTV recordings of those passengers from check-in bags to the departure point.” He would not provide details about what investigators saw in the footage.

Noble, the Interpol secretary general, said it was too soon to speculate about any connection between the stolen passports and the missing plane.

But, he added, “It is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol’s databases.”

Malaysian officials stressed that their priority was locating the aircraft. They said they had reviewed military radar records and raised the possibility that the aircraft tried to turn back just before it lost contact with ground controllers Saturday.

Rodzali Daud, the head of the Royal Malaysian Air Force, said that authorities were “baffled” by the lack of any distress signals from the aircraft but that a closer look at military radar might have indicated a deviation from the flight path.

“We looked into some of the recording on the radar that we have,” he said. “There is a possibility that there was a turn back.”

Mikael Robertsson, the co-founder and co-chairman of Flightradar24, the Stockholm-based service that tracks the majority of the world’s passenger jets, said data gathered by separate, civilian receivers in the region did not appear to show the jet turning around.

“I’m not saying it didn’t turn back, but we can’t see that,” said Robertsson.

Robertsson said a turn made by the aircraft just before it vanished from radar screens was consistent with its flight path.

Aircraft and boats from China, Indonesia, Malaysia, Singapore, Thailand, the United States and Vietnam scoured the area where Malaysian ground controllers lost contact with the plane, the maritime border between Malaysia and Vietnam.