Calum MacLeod, Thomas Maresca and John Bacon

USA TODAY

Military ships and aircraft from a half dozen nations continued to pour into area

The airline has been telling relatives %22to expect the worst%22

Malaysian authorities are investigating up to four passengers with suspect identities

Six planes and seven ships from Vietnam have been unable to find an object spotted by a low-flying plane on Sunday afternoon that could be a piece of the Beijing-bound Malaysia Airlines jet that disappeared early Saturday, authorities said.

Doan Huu Gia, the chief of Vietnam's search and rescue coordination center, said the search for the rectangular piece of debris has proved unsuccessful.

Vietnamese officials said they believe the object is one of the plane's doors, according to local news media reports.

Vietnam civil administration chief Pham Viet Dung said search teams from several countries were sending boats to the area about 56 miles south of Tho Chu island, in an area where an oil slick was spotted Saturday. Authorities said earlier that they had spotted an object in the area that turned out not to be from the plane.

Malaysia Airlines Flight MH370 vanished early Saturday with 239 people aboard, two hours into a scheduled six-hour flight from Kuala Lumpur. A Malaysian official said Sunday the plane may have tried to return before disappearing.

Air force chief Rodzali Daud said military radar indicated the flight "may have made a turn back," but he did not say how far it got. "We are trying to make sense of this," Daud said.

Military ships and aircraft from a half-dozen nations continued searching for the Boeing 777 on Sunday. The U.S. Navy has provided the USS Pinckney, a guided-missile destroyer that carries two MH-60R helicopters, and a P-3C Orion with long-range search, radar and communications capabilities.

"Our thoughts and prayers go out to all of those affected by this tragic event," the Navy said in a statement.

Reuters, citing what it called a senior source involved in the investigation, said the probe is focusing on the possibility that the plane disintegrated in the air.

"The fact that we are unable to find any debris so far appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet," said the source, who is involved in the investigations in Malaysia.

Malaysia Airlines has been telling relatives "to expect the worst," spokesman Ignatius Ong said.

An international team was investigating the crash. American experts include accident investigators from the National Transportation Safety Board and technical experts from the Federal Aviation Administration and Boeing, the NTSB said in a statement.

Authorities were investigating the possibility of terrorism after discovering that two passengers apparently had been flying with stolen passports. Interpol, the France-based international policing agency, confirmed Sunday that the Italian and Austrian passports had been entered into its database after they were reported stolen in 2012 and 2013.

No country had checked the passports with Interpol since the thefts, both of which took place in Thailand, the agency said, adding that it was reviewing the passports of everyone listed on the flight manifest.

"Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, 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," said Interpol Secretary-General Ronald Noble.

The Italian Foreign Ministry said Luigi Maraldi, an Italian national, reported his passport stolen last August. Austrian officials said Christian Kozel's passport was stolen in 2012. Both names were ticketed to continue from Beijing to destinations in Europe and thus did not need visas for China.

Malaysian Transport Minister Hishammuddin Hussein said authorities are looking at four possible cases of suspect identities and had contacted the FBI and other intelligence agencies. "We do not want to target only the four," Hussein said. "We are investigating the whole passenger manifest. We are looking at all possibilities."

When the plane is found, the airline will set up a command center either in Malaysia's Kota Bharu or in Vietnam, depending on its location. A response control center will be activated as close as possible to the incident area, Ong said.

The airline plans to send two family members for each missing passenger to the command center. The airline is working with Chinese authorities to get passports for relatives who lack them, and with the Malaysian Embassy in Beijing to get entry visas for Malaysia.

The first flight will be Monday for relatives who want to travel to Kuala Lumpur, and arrangements will continue for those who decide to wait in Beijing, Ong said.

The 11-year-old jet was last inspected 10 days ago and found in "proper condition," airline officials said. The lack of a distress signal from the pilots "suggests something very sudden and very violent happened," said William Waldock, who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.

Weather was not believed to be a factor. Light rain and snow was falling over South and Central China, but it was well below the aircraft's last known altitude of 35,000 feet.

However the flight disappeared, the mother of Philip Wood, one of three Americans on board, was resigned that he was gone.

"You want to know how it feels to lose a son at the age of 50? It's devastating,'' Sandra Wood said. She saw her son, an IBM executive who worked in Malaysia, a week ago.

Freescale Semiconductor, an Austin-based tech company, said 20 employees from China and Malaysia were aboard. "Our thoughts and prayers are with those affected by this tragic event," CEO Gregg Lowe said.

Subang Air Traffic Control lost contact with the flight at 2:40 a.m. local time (1:40 p.m. ET Friday). It was scheduled to land in Beijing at 6:30 a.m. local time. The last radar signal was received as the aircraft approached Vietnam airspace near the Ca Mau province.

The twin-engine jet was carrying 227 passengers and 12 crewmembers. They're from 14 countries, including 153 from China, 38 from Malaysia, seven from Indonesia, six from Australia, five from India, three from France, two each from New Zealand, Ukraine and Canada and sole travelers from Russia, Italy, Taiwan, Austria and the Netherlands. Besides Wood, the other Americans on the manifest are young children — Nicole Meng, 4, and Yan Zhang, 2.

Malaysia Airlines CEO Ahmad Juahari Yahya said the company is working with emergency responders. "Our thoughts and prayers are with all affected passengers and crew and their family members," Yahya said.

The flight had seasoned pilots, according to the airline. Capt. Zaharie Ahman Shah, 53, of Malaysia has 18,365 flight hours and has been with the airline since 1981. First Officer Fariq Ab Hamid, 27, also of Malaysia, has 2,763 flight hours.

Boeing 777s have a strong safety record. Since their 1995 debut, they've been in only two major accidents.

The worst was last July, when an Asiana Airlines Boeing 777-200 with 291 passengers and 16 crew crashed on landing at San Francisco International Airport. Three passengers were killed, one by a fire rescue truck. There were serious injuries to 48 passengers. Pilot error is being investigated.

The search for the Malaysia Air flight comes amid one of the safer stretches of commercial aviation. In the USA, 2012 was the industry's safest year since the dawn of the jet age. The last major airline disaster was in 2009, when an Air France Airbus 330 crashed during a flight from Rio de Janeiro to Paris, killing all 216 passengers and 12 crew.

Malaysia Air's last air fatalities were in 1995, when a flight crashed near Tawau, Malaysia, killing 34.

MacLeod reported from Beijing, Maresca from Ho Chi Minh City, Vietnam. Contributing: Kim Hjelmgaard in London, Melanie Eversley in New York, Donna Leinwand Leger in Washington, Allison Gray, Doyle Rice and Gary Strauss in McLean, Va., Michael Winter in Oakland; the Associated Press.



