Pete Williams, NBC News

U.S. officials told NBC News on Saturday they are investigating terrorism concerns after two people listed as passengers on the missing Malaysia Airlines jet turned out not to be on the plane and had reported their passports stolen.

The officials said that they had found no clear link to terrorism, and that there are other criminal reasons, for example drug smuggling, that stolen passports might be used to board a plane.

But the revelations, hours after the jet disappeared over the South China Sea without sending a distress signal, significantly changed how U.S. officials looked at the disaster. U.S. officials said they were checking into passenger manifests and going back through intelligence.

"We are aware of the reporting on the two stolen passports," one senior official said. "We have not determined a nexus to terrorism yet, although it's still very early, and that's by no means definitive."

Both passports were stolen in Thailand, sources told NBC News.

An Italian man who had his passport stolen a year ago was on the passenger manifest for the jet, but his father told NBC News on Saturday that he was safe and on vacation in Thailand.

In Austria, the foreign ministry confirmed to NBC News that police had made contact with a citizen who was also on the passenger list, and who reported his passport stolen two years ago while traveling in Asia.

"We believe that the name and passport were used by an unidentified person to board the plane," a spokesman for the ministry said.

It is unusual for one person to board a plane with a stolen passport and very rare for two to do it, terrorism analysts say.

The Italian on the passenger list was Luigi Maraldi, 37. His father, Walter Maraldi, told NBC News from Cesena, Italy: "Luigi called us early this morning to reassure us he was fine, but we didn't know about the accident. Thank God he heard about it before us."

Malaysia has not seen significant terrorist activity, and airport security there has tended to be exemplary.

Asked earlier whether terrorism was suspected in the disappearance of the jet, Malaysian Prime Minister Najib Razak said authorities were "looking at all possibilities," The Associated Press reported.

Earlier in the day, U.S. officials told NBC News that "all we know is something quick and catastrophic" happened to the plane.

The investigation will probably take some time, partly because authorities would have to find wreckage and perform forensics test. A full day after the plane disappeared, there were no signs of the aircraft, although the Vietnamese air force spotted two oil slicks consistent with jet fuel off the coast of Vietnam.

In the crash of TWA Flight 800, in 1996, it took more than a year to rule out terrorism.

Andy Eckardt, Claudio Lavanga, Erin McClam and Michele Neubert of NBC News contributed to this report.