Malaysia intelligence agencies are investigating how up to four passengers with suspect identities were able to board the missing Boeing 777 jet, the government said, as planes and ships from across Asia resumed the hunt Sunday for the plane.

There was still no confirmed sighting of wreckage from the Boeing 777 in the seas between Malaysia and Vietnam where it vanished from screens early Saturday morning en route to Beijing from Kuala Lumpur with 239 people on board. The weather was fine, the plane was already cruising and the pilots had no time to send a distress signal — unusual circumstance for a modern jetliner to crash.

This, and the sudden disappearance of the plane that experts say is consistent with a possible onboard explosion, strengthened existing concerns about terrorism as a possible cause for the disappearance. Al-Qaida militants have used similar tactics to try and disguise their identities.

On Sunday, Malaysian Transport Minister Hishammuddin Hussein said that authorities were now looking at four possible cases of suspect identities. He said Malaysian intelligence agencies were in contact with their international counterparts, including the FBI.

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

“At the same time our own intelligence have been activated, and of course, the counterterrorism units… from all the relevant countries have been informed,” he said.

He added that radar indicated the plane may have turned back before apparently crashing.

Air force chief Rodzali Daud didn’t say which direction the plane might have taken when it apparently went off route.

“We are trying to make sense of this,” he told a media conference. “The military radar indicated that the aircraft may have made a turn back and in some parts, this was corroborated by civilian radar.”

Malaysia Airlines Chief Executive Ahmad Jauhari Yahya said pilots were supposed to inform the airline and traffic control authorities if the plane does start to return. “From what we have, there was no such distress signal or distress call per say, so we are equally puzzled,” he said.

On Saturday, the foreign ministries in Italy and Austria said the names of two citizens listed on the flight’s manifest matched the names on two passports reported stolen in Thailand.

Li Jiaxiang, administrator of the Civil Aviation Administration of China, said some debris had been spotted, but it was unclear whether it came from the plane. Vietnamese authorities said they had seen nothing close to two large oil slicks they saw Saturday and said might be from the missing plane.

Two-thirds of the jet’s passengers were from China. The rest were from elsewhere in Asia, North America and Europe.

More than 30 hours after air traffic control lost communication with the Boeing 777, concerns were mounting over the possible security breach, as authorities in Southeast Asia said there was still no sign of the plane after the search resumed at sea.

Flight MH370 disappeared about an hour after leaving Kuala Lumpur en route to the Chinese capital. A total of 153 Chinese nationals were on board, and anguished relatives camped out at Beijing airport bemoaned the lack of news Sunday.

US officials told The Los Angeles Times that they were trying to determine whether there was any terror link to the flight’s disappearance, but stressed there was no evidence to support that yet.

“Just because they (the passports) were stolen doesn’t mean the travellers were terrorists,” a Department of Homeland Security official told the paper.

“They could have been nothing more than thieves. Or they could have simply bought the passports on the black market.”

Asked to confirm the reports, an FBI spokesman told AFP: “We are ready to assist if needed.”

Vietnamese boats reached the scene of two large oil slicks detected overnight but found no sign of the plane, army deputy chief of staff Vo Van Tuan told AFP early Sunday, adding search aircraft were deployed at first light to scour the area.

The passenger jet slipped off radar screens somewhere between Malaysia’s east coast and southern Vietnam early Saturday morning.

“The airline company didn’t contact me, it was a friend,” a middle-aged woman surnamed Nan told reporters, holding back tears, after finding out her brother-in-law was on the flight.

“I can’t understand the airline company. They should have contacted the families first thing. I don’t have any news. I’m very worried,” she said.

This picture taken on Saturday, March 8, 2014 shows an aerial view of an oil slick taken from a Vietnamese Air Force aircraft taking part in a search mission for a missing Malaysia Airlines aircraft, somewhere between Malaysia's east coast and southern Vietnam.  (photo credit: AFP/Thanh Nien Newspaper)

This picture taken on Saturday, March 8, 2014 shows an aerial view of an oil slick taken from a Vietnamese Air Force aircraft taking part in a search mission for a missing Malaysia Airlines aircraft, somewhere between Malaysia’s east coast and southern Vietnam. (photo credit: AFP/Thanh Nien Newspaper)

However, Azharuddin Abdul Rahman of Malaysia’s Department of Civil Aviation said he “could not confirm” the existence of the oil slicks.

“The rescue operation continued last night until this morning…we have not been able to locate or see anything,” he told a news conference on Sunday.

Asked about the stolen passports, he said, “we are investigating this at the moment” but declined to give further details.

An Austrian named Christian Kozel had his passport stolen in Thailand in 2012, while Italian Luigi Maraldi lost his to theft last year, also in Thailand, officials and reports.

Despite their names being on the passenger manifest, neither man was on the plane to Beijing, officials said.

Earlier, when asked whether terrorism could have been a factor, Malaysian Prime Minister Najib Razak said: “We are looking at all possibilities but it is too soon to speculate.”

‘Pray for flight MH370′

Flight MH370 had relayed no distress signal, indications of rough weather, or other signs of trouble. Both Malaysia’s national carrier and the Boeing 777-200 model used on the route are known for their solid safety records.

The flight was carrying 227 passengers and 12 crew. A US company based in Texas said 20 of its employees were among the missing passengers — a dozen from Malaysia and eight from China.

Thirty-eight Malaysians and seven Indonesians were aboard, as well as a range of other nationalities including Australian, Indian, American, Dutch, and French.

Malaysia Airlines urged “all Malaysians and people around the world to pray for flight MH370.”

A woman wipes her tears after walking out of the reception center and holding area for family and friends of passengers aboard a missing Malaysia Airlines plane, at Kuala Lumpur International Airport in Sepang, Saturday, March 8, 2014. (photo credit: AP Photo/Lai Seng Sin)

A woman wipes her tears after walking out of the reception center and holding area for family and friends of passengers aboard a missing Malaysia Airlines plane, at Kuala Lumpur International Airport in Sepang, Saturday, March 8, 2014. (photo credit: AP Photo/Lai Seng Sin)

The carrier admitted: “It has been more than 24 hours since we last heard from MH370 at 1:30 a.m. The search and rescue team is yet to determine the whereabouts of the Boeing 777-200 aircraft.”

The plane’s disappearance triggered a search effort involving vessels from several nations with rival maritime claims in the tense South China Sea.

China, Vietnam, Malaysia, the Philippines and Singapore threw vessels and aircraft into the effort, as did the US Navy with a surveillance plane and a destroyer carrying two helicopters.

“We are setting these (territorial) issues aside in view of humanitarian reasons,” Philippine military spokeswoman Lieutenant Cheryl Tindog said.

If the worst is confirmed, it would be the second fatal crash in the nearly 20-year history of the popular Boeing plane. A 777-200 operated by South Korea’s Asiana Airlines skidded off the runway after hitting a sea wall in San Francisco last year, killing three people.

Malaysia Airlines has suffered few safety incidents in the past. Its worst occurred in 1977, when 93 people perished in a hijacking and subsequent crash in southern Malaysia.