Ronald K. Noble, the secretary general of the International Criminal Police Organization, or Interpol, said, “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.”

“This is a situation we had hoped never to see,” he said, adding that too few countries systematically screen travelers with Interpol’s Stolen and Lost Travel Documents database set up after the terrorist attacks on Sept. 11, 2001. “For years, Interpol has asked, ‘Why should countries wait for a tragedy to put prudent security measures in place at borders and boarding gates?’ ”

A senior American law enforcement official, who has received classified briefings on the global investigation, said that the authorities had not ruled out terrorism in the plane’s disappearance, but that there had been no public claims of responsibility or electronic intercepts of extremists discussing details of any bombing or attack.

“We’re not seeing or hearing anyone claiming anything about this,” the official said.

By early Monday, the search effort had yet to confirm where the plane might have gone down, even as military aircraft and a flotilla of ships from a half-dozen nations, including China, Malaysia, Vietnam and the United States, searched the waters south of Vietnam.

On Sunday, Vietnamese media reported that rescuers had found a yellow object they thought might be part of the aircraft. But the news media later said it turned out to be a coral reef.

Azharuddin Abdul Rahman, the Malaysian civil aviation chief, said samples from an oil slick discovered in the waters had been collected and were being tested to determine if they had come from the plane.

The flight left the international airport in Sepang, outside Kuala Lumpur, at 12:41 a.m. on Saturday and vanished less than an hour later as it appeared to be cruising at 35,000 feet in calm weather. More details emerged Sunday about the two passengers listed on the manifest using names from an Austrian and an Italian passport reported stolen in Thailand, one in 2012 and the other in 2013. According to electronic booking records, each man bought a one-way ticket on Thursday from a travel agency in a shopping mall in the Thai beach resort of Pattaya. A woman who answered the phone at the agency said she was too busy to talk.

Image A member of the military looked out of a helicopter during a search-and-rescue mission off the Tho Chu Islands of Vietnam on Monday. Credit... Athit Perawongmetha/Reuters

Both men were scheduled to pass through Beijing and continue to Amsterdam before traveling to different cities, Frankfurt and Copenhagen, according to the records.

The senior American law enforcement official confirmed Sunday that Thai officials were investigating a “passport ring” operating on the resort island of Phuket, where both passports were stolen.

Although the official said identifying the two passengers is a top priority for investigators, he noted that false documents were also routinely used in the region by drug smugglers.

Security experts in Asia differed on the significance of the two stolen passports.

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. “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 with stolen passports aboard a single jet was rare.

“It is fairly unusual to have more than one person flying on a flight with a stolen passport,” said Mr. Vickers, who publicly warned 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.”

Mr. Azharuddin said investigators were reviewing video footage of the passengers in question. Malaysian officials also said five ticketed passengers failed to board the flight but said that their luggage was removed from the plane before it took off.