SEPANG, Malaysia — As military aircraft and ships from seven nations combed the waters south of Vietnam on Sunday for signs of a jet with 239 people on board that vanished a day earlier, the authorities here deflected troubling questions about two passengers who had used passports listed in an international database as lost or stolen.
The International Criminal Police Organization, or Interpol, 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. The jet disappeared Saturday en route from Kuala Lumpur to Beijing.
Electronic records emerged linking the passengers with stolen passports to the same travel agency in Thailand. Investigators were working to determine the true identities of those who used the passports.
Aircraft and boats from China, Indonesia, Malaysia, Singapore, Thailand, the United States, and Vietnam scoured the area where ground controllers lost contact with the plane, the maritime border between Malaysia and Vietnam.
Vietnamese aircraft on Sunday spotted what officials suspect was one of the doors of the plane in the same area where an oil slick was seen the day before, the Associated Press reported, citing local media reports. Vietnamese air force officials said it was too dark to confirm the finding.
Azharuddin Abdul Rahman, the Malaysian civil aviation chief, said samples from the oil slick discovered in the waters had been collected and were being tested to determine if it came from the plane.
As the searches continued, an airline official told relatives of the passengers of Flight MH370 to “expect the worst.”
Malaysia Airlines also confirmed that the missing aircraft had been involved in a collision with another plane in 2012 at the Shanghai airport that resulted in damage to the Malaysian aircraft’s wingtip. But the airline said the wing was repaired by Boeing and the plane declared safe to fly.
Security and aviation experts offered starkly disparate theories about what happened to the missing plane, a measure of how unusual it was for it to disappear from radar without any distress call, as well as of how little information the Malaysian authorities have released. The plane appeared to be cruising at 35,000 feet in calm weather when it vanished.
In a series of briefings, Malaysian officials refused to answer questions relating to what they described as “security matters.” “We will review all security protocols and, if needed, we will enhance them,” Prime Minister Najib Razak said.
In China, home to a majority of those aboard the flight, there were signs of anger and frustration over Malaysia’s handling of the situation.
The flight left the international airport in Sepang, outside Kuala Lumpur, at 12:41 a.m. Saturday and disappeared less than an hour later. Speculation on the plane’s fate has ranged from a rare, catastrophic mechanical failure to something more sinister.
Malaysian officials emphasized that their priority was locating the aircraft. They said they had reviewed military radar records and raised the possibility that the aircraft had tried to turn back just before contact with ground controllers was lost.
General Rodzali Daud, the commander of the Royal Malaysian Air Force, said the 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.
More details emerged Sunday about two passengers using names from an Austrian and an Italian whose passports were reported stolen in Thailand, one in 2012 and the other in 2013. According to electronic booking records, both men purchased one-way tickets Thursday from a travel agency in a shopping mall in the Thai resort of Pattaya.
It is unclear how the men traveled from Thailand to Malaysia to board the flight Saturday. But they were both scheduled to transit in Beijing and continue to Amsterdam before traveling to different cities, Frankfurt and Copenhagen, according to the records.
Security experts in Asia said the use of false travel documents was a persistent problem in the region but differed on the significance of the two stolen passports to the investigation.
Xu Ke, a lecturer at the Zhejiang Police College in eastern China who studies aviation safety and hijackings, said the two men might have been illegal migrants.
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 with stolen passports aboard a single jet was rare and a potential clue.
Azharuddin said investigators were reviewing video footage of the passengers in question. “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 declined to provide details about what investigators saw.