Emergency teams expanded their search early Sunday for a Malaysia Airlines flight that is presumed to have crashed in the Gulf of Thailand off Vietnam with 239 people aboard, including four that the Malaysian government said may have boarded with false documents, according to reports.

In a search operation involving at least a half-dozen nations that’s now lasted for more than a day, authorities have turned up no clear signs of wreckage, but Malaysia Airlines said it was “fearing the worst.”

The information about the suspect passengers has led to speculation about terrorism and added to the mystery of Malaysia Airlines Flight MH370, a red-eye carrying passengers from Kuala Lumpur, Malaysia, to Beijing that vanished from radar after midnight Saturday (Friday afternoon EST).

Saturday European officials indicated two of the people on board were using passports that had been stolen in Thailand. On Sunday Malaysia’s transport minister, Hishammuddin Hussein, said Malaysian intelligence officials were also checking the identities of two other passengers, according to The Associated Press.

“All the four names are with me and have been given to our intelligence agencies,” Hishammuddin said, according to The AP. “We do not want to target only the four; we are investigating the whole passenger manifest. We are looking at all possibilities.”

Details about the missing Malaysia Airlines Boeing 777

The mystery deepened on Sunday as Malaysia said the flight might have turned back from its scheduled route to Beijing before disappearing.

“What we have done is actually look into the recording on the radar that we have and we realized there is a possibility the aircraft did make a turnback,” Rodzali Daud, the Royal Malaysian Air Force chief, told reporters at a news conference, according to the Reuters news agency.

Malaysia said it had now expanded its search to the country’s western coast, the opposite side of the peninsular from the plane’s last sighting.

The Vietnamese government said in a statement that two oil slicks spotted off the southern tip of the country were between six and nine miles long and were consistent with what would be left by fuel from a crashed jet, according to the Associated Press.

The National Transportation Safety Board said Saturday night in Washington that a team of investigators was en route from the United States to Asia to assist with the investigation.

As the search resumed Sunday, the airline posted a notice saying that it was “still unable to detect the whereabouts of the missing aircraft,” a Boeing 777-200.

The airline said it would establish a command center either in Kota Bharu, Malaysia, or Ho Chi Minh City, Vietnam, as soon as the location of the aircraft is established. A 94-person caregiver team was providing emotional support for families, the airline said, and an additional team was on the way to Beijing.

A search is launched after contact lost with Malaysia Airlines plane en route from Kuala Lumpur to Beijing. (Reuters)

There was no distress signal from the plane’s pilots, and crashes usually happen during takeoff or landing. That heightened concerns about reports that passengers listed in the airline’s manifest were not on the flight. There were no immediate reports on whether the suspect passengers were seated with one another.

The men, one from Italy and the other from Austria, had reported to authorities that their passports had been stolen in Thailand.

“We are aware of the stolen passport issue and are carrying out an investigation,” Azharuddin told reporters.

Asked earlier whether terrorism was suspected in the plane’s disappearance, Malaysian Prime Minister Najib Razak said authorities were “looking at all possibilities, but it is too early to make any conclusive remarks.”

U.S. officials said Sunday that the cause of the crash remained unclear but that intelligence agencies were examining the possibility of a connection to terrorism.

There were no reports of bad weather in the area.

The plane carried passengers from 14 countries, including three Americans, according to the manifest posted on the airline’s Web site. They were identified as Philip Wood, 51, an IBM employee working in Malaysia; Nicole Meng, 4; and Yan Zhang, 2.

In a brief interview, Wood’s mother, Sondra Wood of Keller, Tex., said she had received a call from the U.S. Embassy in Malaysia. Her son had just been in Texas visiting her and her husband, she said, and she knew he would be on the Malaysia Airlines flight.

“He was a wonderful person and very intelligent,” she said. “I could talk forever about him. He’s my son, and any mother would be proud of their son.”

Aside from his work at IBM, Sondra Wood said, her son loved building furniture. “He was very artistic,” she said. Philip Wood has two sons, ages 20 and 24, his mother said.

Austin-based Freescale Semiconductor confirmed Saturday that 20 of its employees were aboard the plane. Twelve are from Malaysia and eight are from China, the firm’s president and chief executive, Gregg Lowe, said in a statement.

“At present, we are solely focused on our employees and their families,” Lowe said. “Our thoughts and prayers are with those affected by this tragic event.”

Vessels and planes from Southeast Asia have been scouring the waters in the part of the ocean where the oil slicks were spotted, and Razak said “the search-and-rescue operations will continue as long as necessary.”

The Philippines and Singapore sent planes to help in the search, while vessels were dispatched from the Philippines and China, news agencies reported. Vietnamese fishermen were also put on alert.

U.S. 7th Fleet officials said in a statement that the USS Pinckney, a guided-missile destroyer, and a P-3C Orion aircraft were being sent to help in the search.

Meanwhile, there were questions about the identities of two passengers after evidence emerged that they could have been traveling with stolen passports.

Italian news media had initially listed Luigi Maraldi, 37, among the passengers, but he reportedly phoned his parents Saturday to say he was safe in Thailand. His passport had been stolen there last year, the reports said, and he had been issued new documents.

“One hypothesis, therefore, is that he was listed because someone boarded the plane using his stolen passport,” the Corriere della Serra newspaper reported.

Similarly, Austrian news media reported that an Austrian citizen had been listed as among the passengers but had been found safe. His passport was stolen in Thailand two years ago, the Austrian Foreign Ministry said.

Flight MH370 lost contact with Malaysian air traffic control at 1:20 a.m. Saturday (12:20 p.m. EST Friday), less than an hour after taking off from Kuala Lumpur and as it was completing its ascent. It vanished on the border of the territorial waters of Malaysia and Vietnam, where the Gulf of Thailand meets the South China Sea. It had been due to land in Beijing at 6:30 a.m. Saturday (5:30 p.m. EST Friday).

In Beijing, relatives and friends of those on board were taken by minibus from the airport to a hotel in the city to wait for news. Grief was mixed with anger at the lack of information, with Malaysia Airlines insisting it was still investigating the incident. Earlier, it had cited speculation that the plane might have landed in Vietnam, but that was later denied.

The airline said it had sent a team from Malaysia to the hotel in Beijing to look after the relatives. It said it would pay for immediate family members of passengers to gather at the Kuala Lumpur International Airport.

The plane’s sudden disappearance without a call for help brought back memories of an Air France flight that disappeared over the Atlantic Ocean en route from Rio de Janeiro to Paris on June 1, 2009.

While some wreckage and bodies were found in subsequent weeks, it took nearly two years for the main wreckage and the plane’s flight recorders to be recovered. The final report said that pilot errors in responding to technical problems led to the crash.

“We are doing everything in our power to locate the plane. We are doing everything we can to ensure every possible angle has been addressed,” Malaysian Transport Minister Hishammuddin Hussein told reporters near the Kuala Lumpur airport, according to Reuters. “We are looking for accurate information from the Malaysian military. They are waiting for information from the Vietnamese side.”

The South China Sea is a tense region of competing territorial claims among a number of countries, but the plane disappeared well away from the disputed waters, and countries in the region appeared to put aside their differences in their search for the plane.

“In times of emergencies like this, we have to show unity of efforts that transcends boundaries and issues,” said Lt. Gen. Roy Deveraturda, commander of the Philippine military’s Western Command, according to the Associated Press.

Barnes reported from Washington. Harlan reported from Seoul. Liu Liu, Gu Jinglu and Xu Jing in Beijing and Karen DeYoung and Ian R. Shapira in Washington contributed to this report.