Using a system that looks for flashes around the world, the Pentagon reviewed preliminary surveillance data from the area where the plane disappeared and saw no evidence of an explosion, said an American government official who spoke on the condition of anonymity because the subject matter is classified. A team of aviation experts led by the National Transportation Safety Board was on its way to the area.

If all aboard were killed, it would be the deadliest commercial airline accident since Nov. 12, 2001, when an American Airlines Airbus crashed just after takeoff from Kennedy Airport en route to the Dominican Republic.

A senior American intelligence official said law enforcement and intelligence agencies were investigating the issue of the stolen passports. American authorities were scrutinizing the flight manifest closely, the official said, noting that forged travel documents are also used routinely by smugglers and illegal immigrants.

“At this time, we have not identified this as an act of terrorism,” said the official, who spoke on the condition of anonymity Saturday because of the continuing inquiry. “While the stolen passports are interesting, they don’t necessarily say to us that this was a terrorism act.”

A European counterterrorism official said the Italian man whose passport was stolen, Luigi Maraldi, 37, called his parents from Thailand, where he is vacationing, after discovering that someone by the same name was listed on the passenger manifest. The official, who spoke on the condition of anonymity, said Mr. Maraldi reported the theft last August to the Italian police. The official said the passport of the Austrian man, Christian Kozel, 30, who is currently in Austria, was stolen about two years ago.

Image Oil slicks on the surface of the Gulf of Thailand may have been from a crash. Credit... Agence France-Presse — Getty Images

The European official said that he was surprised it had been possible to check in with stolen passports at the Kuala Lumpur airport and that an alert should have popped up on the airline agent’s computer.

Operating as Flight MH370, the plane left Kuala Lumpur just after midnight, headed for Beijing. Air traffic control in Subang, a suburb of Kuala Lumpur, lost contact with the plane around 1:30 a.m. on Saturday, Malaysia’s civil aviation department said.

China Central Television said that according to Chinese air traffic control officials, the aircraft never entered Chinese airspace.

At a news conference in Beijing after the arrival of a team of employees to assist families of the passengers in China, an official of Malaysia Airlines said the missing plane had no history of malfunctions. “It was last inspected 10 days ago, well before scheduled service,” said the executive, Ignatius Ong. “It was all in top condition.”

When pressed about possible security lapses, he repeated several times that the airline had no confirmation from the Malaysian authorities that passengers had boarded with stolen passports.

Image Ahmad Jauhari Yahya, the chief executive of Malaysia Airlines, addressed the media as the search for the plane continued. Credit... Samsul Said/Reuters

Malaysia, the United States and Vietnam dispatched ships and aircraft to the mouth of the Gulf of Thailand on Saturday to join an intensive search, and the state-run Xinhua news agency said China was sending a Coast Guard vessel and two naval ships. The Chinese Ministry of Transport said a team of scuba divers who specialize in emergency rescues and recovery had been assembled on Hainan, the southern island-province, to prepare to go on Sunday to the area where the airliner may have gone down.

Lai Xuan Thanh, the director of the Civil Aviation Administration of Vietnam, said on Saturday that a Vietnamese Navy AN26 aircraft had discovered oil on the surface of the water toward the Vietnam side of the mouth of the Gulf of Thailand, east of the Malaysian Peninsula. The oil is suspected to have come from the missing plane, he added.

But on Sunday, Malaysia’s defense minister, Hishammuddin Hussein, added a confusing twist, saying the plane might have gone down west of the Malaysian peninsula. “We are looking at the possibility of an aircraft air turnback,” he said without elaborating whether the plane might have changed course for mechanical reasons or a hijacking, or why the authorities suspected the plane might have reversed course.

Malaysia Airlines said the plane had 227 passengers aboard, including two toddlers, and an all-Malaysian crew of 12. According to the manifest, the passengers included 154 citizens of China or Taiwan, 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans, as well as two citizens each from Canada, New Zealand and Ukraine and one each from Austria, Italy, the Netherlands and Russia — although the true nationalities of the passengers carrying the Austrian and Italian passports are still unknown.

The family of one of the Americans aboard the flight, Philip Wood, an IBM employee in Kuala Lumpur, said they had little information beyond what had been reported in the news media.

Image A distraught couple at the Beijing Capital International Airport wait for news about the missing Malaysia Airlines flight. Credit... Kim Kyung-Hoon/Reuters

“We’re relying on our Lord,” Mr. Wood’s father, Aubrey, said from his home in Keller, Tex. “He’s the one who carries the load.”

The tickets to the holders of the stolen Austrian and Italian passports were sold by China Southern Airlines, which has a code share agreement with Malaysia Airlines, according to China Southern’s account on Sina Weibo, the Chinese microblog platform. China Southern said it sold five other tickets to the flight: to the Dutch passenger, the Ukrainians, and one Malaysian and one Chinese passenger.

Arnold Barnett, a longtime Massachusetts Institute of Technology specialist in aviation safety statistics, said that before the disappearance of the plane, Malaysia Airlines had suffered two fatal crashes, in 1977 and 1995. Based on his estimate that Malaysia Airlines operates roughly 120,000 flights a year, he calculated that the airline’s safety record was consistent with that of airlines in other fairly prosperous, middle-income countries, but had not yet reached the better safety record of airlines based in the world’s richest countries.

Malaysia has not been targeted in terrorist attacks in recent decades, although the 1977 crash was attributed to a hijacking. But some of the planning for the Sept. 11 attacks in the United States was done in Malaysia, which has a relatively lax visa policy. The country is a major trading nation and a natural meeting place for a variety of groups involved in illicit activities.

The plane’s disappearance came at the end of the annual National People’s Congress in Beijing, and took place at a time of rising concern in China about terrorism. Mr. Ahmad Jauhari of Malaysia Airlines said early on in a statement that there was speculation that the plane had landed safely somewhere along the route to Beijing. But in a telephone interview before reporting the sighting of the oil in the ocean, Mr. Lai expressed concern about the aircraft’s fate.

“The possibility of an accident is high,” he said.