Officials say two passengers on the missing Malaysia Airlines jet were traveling with stolen passports.
Two men listed on the passenger list -- one from Italy and another from Austria -- never boarded the flight from Kuala Lumpur to Beijing that disappeared early Saturday. Two-hundred-and-39 people were on board.
Both men had their passports stolen in Thailand in the last two years. It is not clear who was flying with the stolen documents.
U.S. officials say they are still looking at the disappearance as if it is an accident, but Malaysian officials say they are not ruling anything out.
Rescue pilots from Vietnam say they spotted two large oil slicks in the waters off that country's southwestern coast, but no debris has been seen.
Ships and planes from China, the Philippines, and the United States have also joined in the search.
Malaysia Airlines Flight MH370 disappeared from radar screens about an hour after taking off from Kuala Lumpur for Beijing. Most of the passengers were Chinese.
Air traffic controllers say they never received any distress calls before the jet disappeared.
The aircraft is a Boeing 777-200 -- a very popular plane with an excellent safety record. The most recent accident involving a Boeing 777 was the Asiana Airlines crash at San Francisco International Airport in July 2013. Three people were killed. Pilot error is suspected in that incident.