Two large oil slicks spotted Saturday by the Vietnamese air force offered the first sign that a jetliner carrying 239 people had crashed into the ocean after vanishing from radar without sending a single distress call.
An international fleet of planes and ships scouted the waters between Malaysia and Vietnam for any clues to the fate of the Malaysian Airlines Boeing 777, which disappeared less than an hour after taking off from Kuala Lumpur bound for Beijing.
The oil slicks sighted off the southern tip of Vietnam were each between 10 kilometers (6 miles) and 15 kilometers (9 miles) long, the Vietnamese government said in a statement.
There was no immediate confirmation that the slicks were related to Flight MH370, but the government said they were consistent with the kind of slick that would be produced by the jet's two fuel tanks.
After the oil was spotted, authorities suspended the air search for the night. It was to resume Sunday. A sea search continued in the darkness, the airline said.
The jet's disappearance was especially mysterious because it apparently happened when the plane was at cruising altitude, not during the more dangerous phases of takeoff or landing.
Just 9 percent of fatal accidents happen when a plane is at cruising altitude, according to a statistical summary of commercial jet accidents done by Boeing.
Malaysia Airlines CEO Ahmad Jauhari Yahya said there was no indication the pilots had sent a distress signal. That might mean that whatever trouble befell the plane happened so fast the crew did not have time to broadcast even a quick mayday.
The lack of a radio call "suggests something very sudden and very violent happened," said William Waldock, who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.
The plane was last inspected 10 days ago and found to be "in proper condition," Ignatius Ong, CEO of Malaysia Airlines subsidiary Firefly airlines, said at a news conference.
Two-thirds of the jet's passengers were from China. The rest were from elsewhere in Asia, North America and Europe.
Asked whether terrorism was suspected, Malaysian Prime Minister Najib Razak said authorities were "looking at all possibilities, but it is too early to make any conclusive remarks."
Contributing to fears of foul play was word from foreign ministries in Italy and Austria that the names of two citizens listed on the flight's manifest matched the names on two passports reported stolen in Thailand.
Italy's Foreign Ministry said an Italian man who was listed as being a passenger, Luigi Maraldi, was traveling in Thailand and was not aboard the plane. It said he reported his passport stolen last August.
Austria's Foreign Ministry confirmed that a name listed on the manifest matched an Austrian passport reported stolen two years ago in Thailand. It said the Austrian was not on the plane, but would not confirm the person's identity.
The amount of time needed to find aircraft that go down over the ocean can vary widely. Planes that crash into relatively shallow areas, like the waters off Vietnam where the Malaysian jet is missing, are far easier to locate and recover than those that plunge deep into undersea canyons or mountain ranges.
At Beijing's airport, authorities posted a notice asking relatives and friends of passengers to gather at a nearby hotel to await further information. A woman aboard a shuttle bus wept, saying on a mobile phone, "They want us to go to the hotel. It cannot be good."
Passengers' loved ones were escorted into a private area at the hotel, but reporters were kept away. A man in a gray hooded sweatshirt later stormed out complaining about a lack of information. The man, who said he was a Beijing resident but declined to give his name, said he was anxious because his mother was aboard the flight with a tourist group.
"We have been waiting for hours and there is still no verification," he said.
In Kuala Lumpur, family members gathered at the airport, but were also kept away from reporters.
The plane was last seen on radar at 1:30 a.m. (1730 GMT Friday) above the waters where the South China Sea meets the Gulf of Thailand, authorities in Malaysia and Vietnam said.
Lai Xuan Thanh, director of Vietnam's civil aviation authority, said air traffic control in the country never made contact with the plane.
The South China Sea is a tense region with competing territorial claims that have led to several low-level conflicts, particularly between China and the Philippines. That antipathy briefly faded Saturday as China, the Philippines, Vietnam, Singapore and Malaysia all sent ships and planes to the region.
Malaysia had dispatched 15 planes and nine ships to the area, Najib said.
The U.S. Navy was sending a warship and a surveillance plane, while Singapore said it would send a submarine and a plane. China and Vietnam also sent aircraft to help in the search.
The plane was carrying 227 passengers and 12 crew members, the airline said. It said there were 152 passengers from China, 38 from Malaysia, seven from Indonesia, six from Australia, five from India, three from the U.S., and others from Indonesia, France, New Zealand, Canada, Ukraine, Russia, Taiwan and the Netherlands.
Malaysia Airlines has a good safety record, as does the 777, which had not had a fatal crash in its 19-year history until an Asiana Airlines plane crashed last July in San Francisco, killing three passengers, all teenagers from China.
If wreckage is found, a top priority will be recovering the airliner's "black boxes," the flight-data and cockpit-voice recorders that are equipped with "pingers" that emit ultrasonic signals that can be detected underwater. Under good conditions, the signals can be detected from several hundred miles away, said John Goglia, a former member of the U.S. National Transportation Safety Board.
If the boxes are trapped inside the wreckage, the sound may not travel as far. If the boxes are at the bottom of an underwater trench, that also hinders how far the sound can travel. Signals can weaken over time.
When Air France Flight 447, flying from Rio de Janeiro to Paris, went down in the Atlantic with 228 people on board, the Airbus 330 fell into especially deep water on June 1, 2009. It took nearly two years to recover the main wreckage and the black boxes from a depth of around 13,000 feet.
By contrast, much of the Gulf of Thailand is less than 300 feet deep.
The 53-year-old pilot of Flight MH370, Zaharie Ahmad Shah, has more than 18,000 flying hours and has been flying for the airline since 1981. The first officer, 27-year-old Fariq Hamid, has about 2,800 hours of experience and has flown for the airline since 2007,Malaysia Airlines said.
A wingtip on the same Malaysian Airlines Boeing 777-200 broke off Aug. 9, 2012, as it was taxiing at Pudong International Airport outside Shanghai. The wingtip had collided with the tail of a China Eastern Airlines A340 plane. No one was injured.