HONG KONG — A 12-mile-long streak of oil across the surface waters of the Gulf of Thailand was an early clue to the mysterious disappearance of a Malaysia Airlines jet with 239 aboard that vanished in predawn darkness Saturday morning during a flight from Kuala Lumpur that was supposed to end in Beijing.  But as the sun set over the gulf and the adjacent South China Sea on Saturday, the disappearance of the plane was a reminder that even the most modern planes can suddenly and disconcertingly disappear with few traces. In 2009, an Air France Airbus 330 slipped off radar screens into the deep waters of the Atlantic off Brazil, another case in which the wreckage proved difficult to find.  As of Saturday evening, the Malaysian plane, a Boeing 777-200 on Flight MH370, had not yet been confirmed to have crashed, though the limits of its fuel tanks mean that it came down somewhere instead of reaching Beijing at dawn on Saturday. The Gulf of Thailand, if that is where the plane ended up, has one advantage for rescuers in that it is a shallow arm of the South China Sea, with no comparison to the inky depths of the Atlantic.  Malaysia’s deputy minister of transport, Aziz bin Kaprawi, said the authorities had not received any distress signals from the aircraft.  In a development that raised fears of foul play, foreign ministry officials in Vienna and Rome confirmed that the names of two citizens, an Italian and an Austrian, listed on the manifest of the missing flight matched the names on two passports reported stolen in Asia, news reports said. The Italian man, Luigi Maraldi, told the Italian news media that he was currently in Bangkok, and was not the Luigi Maraldi listed on the plane’s manifest. An Austrian Foreign Ministry spokesman would not identify the Austrian.  “We are not ruling out anything,” the chief executive of Malaysia Airlines, Ahmad Jauhari Yahya, told reporters at Kuala Lumpur International Airport on Saturday night. “As far as we are concerned right now, it’s just a report.”  A senior American intelligence official said law enforcement and intelligence agencies were investigating the matter. But so far, they had no leads.  “At this time, we have not identified this as an act of terrorism,” said the official, who spoke on the condition of anonymity because of the continuing inquiry. “While the stolen passports are interesting, they don’t necessarily say to us that this was a terrorism act.”  Xinhua, the Chinese state news agency, reported that the Chinese prime minister, Li Keqiang, held an urgent telephone call with his Malaysian counterpart, Najib Razak, telling him, “The urgent task now is to quickly clarify the situation, and use a range of means to enhance the intensity of search and rescue.”  Malaysia Airlines said the plane had 227 passengers aboard, including two infants, and an all-Malaysian crew of 12. The passengers included 154 citizens from 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.  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. China said it had sent a vessel to the area at top speed that would arrive there on Sunday afternoon.  Lai Xuan Thanh, the director of the Civil Aviation Administration of Vietnam, said a Vietnamese Navy AN26 aircraft had discovered the oil slick toward the Vietnam side of the mouth of the Gulf of Thailand.  Fredrik Lindahl, the chief executive of Flightradar24, an online aircraft tracking service, said the missing plane had been equipped with a transponder that regularly transmitted its position via GPS satellites. The last recorded position of Flight MH370 was 93 miles northeast of Kuala Terengganu, a port on the northeast coast of Peninsular Malaysia, he wrote in an email.  Mr. Ahmad of Malaysia Airlines said in a statement that there had been speculation that the plane landed safely somewhere along the route to Beijing, and said the airline was investigating. But in a telephone interview before reporting the sighting of the slick, Mr. Lai expressed concern about the aircraft’s fate.  “The possibility of an accident is high,” he said.  Relatives of those on the missing flight who were waiting at Beijing Capital International Airport were taken to a hotel and kept waiting in a room for hours, prompting complaints. One woman said no one from Malaysia Airlines had come to the room to talk to relatives.  Liu Meng, 26, who works for a communications company, said he had been waiting for his boss to arrive from Malaysia since 6 a.m. “I was able to contact him up until yesterday afternoon,” Mr. Liu said. “After that, nothing.”  At the Kuala Lumpur airport, a grief-stricken relative of a passenger aboard MH370 screamed uncontrollably as he was escorted out of the terminal by airline employees.  “Be truthful about this!” said the man, Koon Chim Wa, whose booming voice echoed through the cavernous terminal.  “They say they don’t know where the plane is,” Mr. Koon said, his hands and body shaking. “Is this a joke?”  Lt. Col. Pham Hong Soi, the head of the propaganda department of the Vietnam Navy for the region near the crash site, said one rescue vessel had already been ordered to sea and two more were ready for departure.  Malaysia’s prime minister, Mr. Najib, said in a statement that 15 aircraft and nine ships were searching for the missing plane. Without saying where his government suspected that the plane disappeared, he added, “Our priority now is to widen the search area and provide support to relatives of those missing.”  The United States Seventh Fleet said it was sending a destroyer, the Pinckney, and a P-3C maritime surveillance aircraft to join the search for Flight MH370.  In addition, the Chinese State Oceanic Administration said it had sent a Coast Guard ship to the area where the plane might have gone down. “It is traveling at full speed to the waters, and is expected to reach there on the afternoon of the 9th,” said a statement on the administration’s website.  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 where airliner may have gone down.  China Central Television said that according to Chinese air traffic control officials, the aircraft never entered Chinese airspace.  Boeing said in a statement that it was assembling a team of technical experts to advise the national authorities investigating the disappearance of the aircraft.  One uncertainty about the flight involved the timing of its disappearance from radar. Malaysia Airlines said it took off at 12:41 a.m. Malaysia time and disappeared from air traffic control radar in Subang, a suburb of Kuala Lumpur, at 2:40 a.m.  That timeline seemed to suggest that the plane stayed in the air for two hours — long enough to fly not only across the Gulf of Thailand but also far north across Vietnam. But Mr. Lindahl of Flightradar 24 said that the last radar contact had been at 1:19 a.m., less than 40 minutes after the flight began.  A Malaysia Airlines spokesman said on Saturday evening that the last conversation between the flight crew and air traffic control in Malaysia had been around 1:30 a.m., but he reiterated that the plane had not disappeared from air traffic control systems in Subang until 2:40 a.m.  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, near the Equator, is a popular winter vacation destination for affluent residents of chilly, smoggy Beijing.  The list of Chinese passengers aboard the missing flight included the names of artists who had attended a meeting in Kuala Lumpur, said the newspaper Beijing News. Other reports said the missing included members of a returning delegation of Buddhists.  If the plane is confirmed to have crashed into the sea, the disaster would add to a difficult week for China and its government. Last Saturday, a group of assailants used knives and daggers to kill 29 people and wound more than 140 at a train station in Kunming, a city in southwest China.  China’s growing wealth has brought a steep rise in the number of its citizens traveling overseas, especially throughout Asia, and the government has sometimes faced allegations, especially from Internet users, that officials failed to adequately help victims of emergencies abroad and their families. This time, President Xi Jinping of China and other senior officials swiftly issued statements to show they were closely following developments.  Because of an editing error, an earlier version of this article referred incorrectly, on second reference, to the chief executive officer of Malaysia Airlines, Ahmad Jauhari Yahya. He is Mr. Ahmad, not Mr. Yahya.  Reporting was contributed by Thomas Fuller from Kuala Lumpur, Malaysia; Eric Schmitt from Washington; Chau Doan from Hanoi, Vietnam; Amy Qin from Beijing; Chris Buckley from Hong Kong; and Alison Smale from Berlin. Bree Feng contributed research from Beijing.
A 12-mile-long oil slick spotted between Malaysia and Vietnam is the first sign that a missing Malaysia Airlines jet carrying 239 people went down in the waters between the two countries.