Oil slicks signaled that a missing Malaysia Airlines jet may have crashed in the Gulf of Thailand even as the mystery surrounding the plane deepened with the discovery that two passengers used stolen passports.

An air search was resuming Sunday after Vietnam's military found twin sheens as long as 15 km off the country's south coast Saturday. That marked the first clue in the disappearance of Flight 370 and the 239 people on board the Boeing Co. 777-200 en route to Beijing from Kuala Lumpur.

Six nations hunting for the plane had little to go on, with no distress calls, emergency-beacon signals, bad weather or other signs why a cruising airliner would lose touch in one of the safest phases of flight. The prospect of terrorism arose after Austria and Italy said that two passengers used passports stolen from their nationals, both of them men.