BEIJING — A massive search was underway Sunday for a missing Malaysia Airlines jetliner, focusing on a spot off the southern coast of Vietnam where two large oil slicks were reported. But there were, so far, no clues to why the China-bound flight vanished without warning with 239 people on board.

Malaysian officials investigating the disappearance said they were not ruling out terrorism — or any other causes — as reports emerged that two Europeans listed on the passenger manifest were not aboard and their passports had been lost or stolen. Who was traveling on those passports, though, remained unclear.

At a news conference Sunday morning, Azharuddin Abdul Rahman of Malaysia's Department of Civil Aviation said searchers had "not located anything" and were expanding the search area. He said even the oil slick sighting, reportedly made by a Vietnamese military plane, had not been verified.

Photos: Missing Malaysia Airlines jet

In Washington, the FBI said it was opening its own investigation into the plane's disappearance, but officials cautioned that there was no immediate evidence that pointed to terrorism.

"So far, what happened is a mystery," a top U.S. law enforcement official said, speaking on condition of anonymity because he was not authorized to speak about the investigation.

The Boeing 777 was carrying 227 passengers and a dozen Malaysian crew members when it lost contact with air traffic controllers about 2:40 a.m. Saturday, two hours after departing Kuala Lumpur, Malaysia, en route to Beijing, the airline said. The biggest contingent — 154 — was from China and Taiwan.

The airline's chief executive, Ahmad Jauhari Yahya, said during a news conference in Kuala Lumpur that there was no distress call or bad weather report from the pilots before the plane lost contact with air control 140 miles off the east coast of Kota Bharu, Malaysia.

Three American passport holders were listed among the passengers, and a Texas semiconductor company said 20 of its employees —12 from Malaysia and eight from China — were aboard.

"At present, we are solely focused on our employees and their families," said Gregg Lowe, president and chief executive of Austin-based Freescale Semiconductor. "Our thoughts and prayers are with those affected by this tragic event."

Ships from Malaysia, China and the United States were en route to the suspected crash site early Sunday.

Video: Relatives fraught after jet disappears

Two Chinese warships, the Jinggangshan and Mianyang, were deployed to the area, state-run New China News Agency said. The Jinggangshan carried helicopters, medical personnel and divers.

The U.S. Navy's 7th Fleet said it had dispatched the Pinckney, a guided-missile destroyer with helicopters aboard, and was also sending a P-3C Orion aircraft, which has long-range search, radar and communications capabilities.

Azharuddin, Malaysia's director general of civil aviation, said Saturday night that authorities had reviewed closed-circuit TV footage of passengers and their luggage and hadn't seen anything of concern. But Prime Minister Najib Razak cautioned that it was too early to come to any conclusions, and other officials said nothing was being ruled out at this point.

The U.S. law enforcement source said FBI personnel would also take a look at the video and use the bureau's vast counter-terrorism technology to look for matches with known members of Al Qaeda or other terrorist organizations. But he emphasized that no known terrorist link had surfaced and no organization had claimed responsibility for downing the plane.

Speculation swirled around the two passengers who may have been using stolen passports.

The airline released a passenger list that included, among many other nationalities, one Italian and one Austrian. Shortly after the list was published, Italy's ANSA news agency reported that the Italian named on the manifest, Luigi Maraldi, 37, had phoned his family to say he was alive and well in Thailand.

Austria's APA news agency made a similar report about an Austrian citizen listed on the passenger manifest, Christian Kozel, 30. APA reported that his passport was stolen about two years ago in Thailand.

An official at the U.S. Department of Homeland Security said the Italian's passport had been taken from his rental car when he returned the vehicle in August.