Story highlights "We have not been able to locate anything," an airline official says

U.S. law enforcement sources say both passports were stolen in Thailand

One of the two stolen passports is listed in Interpol's database, sources say

Vietnamese searchers spot oil slicks in the South China Sea

There were few answers Sunday about the fate of Malaysia Airlines Flight 370, a day after contact was lost with the commercial jetliner en route from Kuala Lumpur to Beijing.

An aerial search resumed at first light, with aircraft searching an area of the South China Sea for any sign of where the flight may have gone down, Azharuddin Abdul Rahman, the director general of civil aviation in Malaysia, told reporters

"We have not been able to locate anything, see anything," Rahman said. "There's nothing new to report."

The closest things to clues in the search for the missing jetliner are oil slicks in the Gulf of Thailand, about 90 miles south of Vietnam's Tho Chu Island -- the same area where the flight disappeared from radar early Saturday morning. A Vietnamese reconnaissance plane, part of a massive, multinational search effort, spotted the oil slicks that stretch between six and nine miles, the Vietnam government's official news agency reported.

Malaysian authorities have not yet confirmed the Vietnamese report, Rahman said.

The reported oil discovery has only added to a growing list of questions about the fate of the plane carrying 227 passengers and 12 crew members: What happened to the plane, why was no distress signal issued, and who exactly was aboard?

Passenger manifest questioned

Bits and pieces of information have begun to form, but it remains unclear how they fit into the bigger picture, if at all.

For instance, after the airline released a manifest, Austria denied that one of its citizens was aboard the flight. The Austrian citizen was safe and sound, and his passport had been stolen two years ago, Austrian Foreign Ministry spokesman Martin Weiss said.

Photos: The search for MH370 Photos: The search for MH370 Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board. Hide Caption 1 of 43 Photos: The search for MH370 On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight. Hide Caption 2 of 43 Photos: The search for MH370 Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft. Hide Caption 3 of 43 Photos: The search for MH370 In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was. Hide Caption 4 of 43 Photos: The search for MH370 Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead. Hide Caption 5 of 43 Photos: The search for MH370 A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane. Hide Caption 6 of 43 Photos: The search for MH370 Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public. Hide Caption 7 of 43 Photos: The search for MH370 Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014. Hide Caption 8 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014. Hide Caption 9 of 43 Photos: The search for MH370 The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014. Hide Caption 10 of 43 Photos: The search for MH370 A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014. Hide Caption 11 of 43 Photos: The search for MH370 A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014. Hide Caption 12 of 43 Photos: The search for MH370 Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014. Hide Caption 13 of 43 Photos: The search for MH370 A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014. Hide Caption 14 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014. Hide Caption 15 of 43 Photos: The search for MH370 On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370. Hide Caption 16 of 43 Photos: The search for MH370 The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions. Hide Caption 17 of 43 Photos: The search for MH370 A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014. Hide Caption 18 of 43 Photos: The search for MH370 Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014. Hide Caption 19 of 43 Photos: The search for MH370 People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014. Hide Caption 20 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived." Hide Caption 21 of 43 Photos: The search for MH370 Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014. Hide Caption 22 of 43 Photos: The search for MH370 A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014. Hide Caption 23 of 43 Photos: The search for MH370 A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia. Hide Caption 24 of 43 Photos: The search for MH370 Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search. Hide Caption 25 of 43 Photos: The search for MH370 Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight. Hide Caption 26 of 43 Photos: The search for MH370 A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014. Hide Caption 27 of 43 Photos: The search for MH370 On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet. Hide Caption 28 of 43 Photos: The search for MH370 U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014. Hide Caption 29 of 43 Photos: The search for MH370 Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean. Hide Caption 30 of 43 Photos: The search for MH370 A Vietnamese military official looks out an aircraft window during search operations March 13, 2014. Hide Caption 31 of 43 Photos: The search for MH370 Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014. Hide Caption 32 of 43 Photos: The search for MH370 Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014. Hide Caption 33 of 43 Photos: The search for MH370 A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014. Hide Caption 34 of 43 Photos: The search for MH370 A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014. Hide Caption 35 of 43 Photos: The search for MH370 A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand. Hide Caption 36 of 43 Photos: The search for MH370 Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014. Hide Caption 37 of 43 Photos: The search for MH370 Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014. Hide Caption 38 of 43 Photos: The search for MH370 The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014. Hide Caption 39 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014. Hide Caption 40 of 43 Photos: The search for MH370 A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014. Hide Caption 41 of 43 Photos: The search for MH370 Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8, 2014. Hide Caption 42 of 43 Photos: The search for MH370 Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said. Hide Caption 43 of 43

Map: Malaysia airliner lost contact

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Similarly, Italy's foreign ministry confirmed none of its citizens were on Flight 370, even though an Italian was listed on the manifest.

On Saturday, Italian police visited the home of the parents of Luigi Maraldi, the man whose name appeared on the manifest, to inform them about the missing flight, said a police official in Cesena, in northern Italy.

Maraldi's father, Walter, told police he had just spoken to his son, who was fine and not on the missing flight, said the official, who is not authsorized to speak to the media. Maraldi was vacationing in Thailand, his father said.

The police official said Maraldi had reported his passport stolen in Malaysia last August and had obtained a new one. But U.S. law enforcement sources told CNN that both the Austrian and Italian passports were stolen in Thailand.

"No nexus to terrorism yet," a U.S. intelligence official said, "although that's by no means definitive. We're still tracking."

Malaysian authorities have been in contact with counterterrorism organizations about possible passport issues, Malaysia's transportation minister Hishamuddin Hussein said. He did not state how many passport issues there are, saying authorities are looking at the whole manifest.

The U.S. government has been briefed on the stolen passports and reviewed the names of the passengers in question but found nothing at this point to indicate foul play, said a U.S. law enforcement official, who spoke on condition of anonymity.

Of the two passports in question, the Italian one had been reported stolen and was in Interpol's database, CNN Law Enforcement Analyst Tom Fuentes said, citing sources at Interpol.

Additionally, no inquiry was made by Malaysia Airlines to determine if any passengers on the flight were traveling on stolen passports, he said. Many airlines do not check the database, he said.

During the news conference in Kuala Lumpur, Rahman declined to say whether the airline or Malaysian authorities had checked the database.

Not ruling anything out

Malaysian authorities reiterated during a news conference that they are not ruling anything out regarding the missing aircraft.

The Boeing 777-200ER departed Kuala Lumpur International Airport at 12:41 a.m. Saturday in good weather, and it was expected to land in Beijing at 6:30 a.m., a 2,300-mile (3,700-kilometer) trip.

Air traffic controllers in Subang, outside Kuala Lumpur, lost contact with the plane about 1:30 a.m., Rahman said. Earlier, the airline said the jetliner lost contact at 2:40 a.m.

The pilots did not indicate to the tower there may be a problem, and no distress signal was issued, the airline said.

It may be days, possibly weeks or months, before authorities can offer any firm answers.

It took five days for authorities to locate the wreckage of Air France Flight 447 when it crashed June 1, 2009, in the Atlantic Ocean, killing all 228 on board.

It took four searches over the course of nearly two years to locate the bulk of Flight 447's wreckage and the majority of the bodies in a mountain range deep under the ocean.

If Malaysia Airlines Flight 370 went down in the Gulf of Thailand, the recovery may be a bit easier because it is a relatively shallow area of the South China Sea, according to marine officials.

Hussein, who in addition to being Malaysia's acting transportation minister is also its defense minister, said that the search area has been expanded because of the possibility the plane had turned back.

China, Vietnam, Singapore and Malaysia were conducting search and rescue operations south of Tho Chu island in the South China Sea, according to the airline and reports from Xinhua, China's official news agency. Ships, helicopters and airplanes are being utilized.

The USS Pinckney, a destroyer conducting training in the South China Sea, is being routed to the southern Vietnamese coast to aid in the search, the U.S. Navy said. The United States is also sending a P-3C Orion surveillance plane from Japan to provide long-range search, radar and communications capabilities, the Navy said.

Meanwhile, the Chinese Coast Guard has ordered on-duty vessels to aid in the search, Xinhua reported, citing government officials. China also sent a diving and salvage team to the area where the airplane is suspected to have gone down, the news agency reported.

Because of the Americans aboard the flight, the FBI has offered to send a team of agents to Malaysia to support the investigation into the disappearance if asked, a U.S. official familiar with the issue told CNN on condition of anonymity. Earlier, an official had said FBI agents were heading to the area.

The FBI is not ruling out terrorism or any other issue as a possible cause in the jetliner's disappearance, the official said.

Officials appeared resigned to accepting the worst outcome.

"I'd just like to say our thoughts and prayers are with the bereaved families," Malaysian Prime Minister Najib Razak said during a news conference.

Grief, especially in China

The plane carried 227 passengers, including five children under 5 years old, and 12 crew members, the airline said. At the time of its disappearance, the Malaysia Airlines plane was carrying about 7.5 hours of fuel, an airline official said.

Among the passengers there were 154 people from China or Taiwan; 38 Malaysians, and three U.S. citizens.

Relatives of the Chinese citizens on board gathered Saturday at a hotel complex in the Lido district of Beijing as a large crowd of reporters gathered outside.

"My son was only 40 years old," one woman wailed as she was led inside. "My son, my son. What am I going to do?"

Family members were kept in a hotel conference room, where media outlets had no access. Most of the family members have so far refused to talk to reporters. The airline said the public can call 603 7884 1234 for further information.

In Malaysia, the families and loved ones of those aboard the flight were gathered at the Everly Hotel in Putrajaya, south of Kuala Lumpur, according to Bernama, the Malaysian national news agency.

Twenty of the passengers aboard the flight work with Freescale Semiconductor, a company based in Austin, Texas. The company said that 12 of the employees are from Malaysia and eight are from China.

The airline's website said the flight was piloted by a veteran.

Capt. Zaharie Ahmad Shah, a 53-year-old Malaysian, has 18,365 total flying hours and joined Malaysia Airlines in 1981, the website said. The first officer is Fariq Ab Hamid, 27, a Malaysian with a total of 2,763 flying hours. He joined Malaysia Airlines in 2007.

Still an 'urgent need' to find plane

"The lack of communications suggests to me that something most unfortunate has happened," said Mary Schiavo, former inspector general of the U.S. Department of Transportation, in an interview with CNN International.

"But that, of course, does not mean that there are not many persons that need to be rescued and secured. There's still a very urgent need to find that plane and to render aid," she said.

Malaysia Airlines operates in Southeast Asia, East Asia, South Asia, the Middle East and on the route between Europe and Australasia.

It has 15 Boeing 777-200ER planes in its fleet, CNN's Richard Quest reported. The missing airplane was delivered to Malaysia Airlines in 2002.

Part of the company is in the private sector, but the government owns most of it.

Malayan Airways Limited began flying in 1937 as an air service between Penang and Singapore. A decade later, it began flying commercially as the national airline.

In 1963, when Malaysia was formed, the airline was renamed Malaysian Airlines Limited.

Within 20 years, it had grown from a single aircraft operator into a company with 2,400 employees and a fleet operator.

If this aircraft has crashed with a total loss, it would be the deadliest aviation incident since November 2001, when an American Airlines Airbus A300 crashed in Belle Harbor, Queens, shortly after takeoff from JFK Airport. Killed were 265 people, including five people on the ground.