An extensive search has revealed no sign of the missing Malaysia Airlines plane. What authorities say is known as of Monday evening:

- The Boeing 777 carrying 239 people lost contact over the sea between Malaysia and Vietnam; there was no sign of trouble before it disappeared early Saturday, and no distress signal was sent.

- At least 34 aircraft and 40 ships from several countries are searching a 50-nautical mile radius from the point the plane vanished, but the only finds have been false alarms - a yellow object spotted by a search plane turned out to be trash, and oil slicks were shown to not be from an aircraft.

- Police and Interpol questioned the proprietors of a travel agency in Thailand that sold one-way tickets to two men who traveled on stolen passports.

What is not yet known:

- What happened to cause the plane to lose contact. Catastrophic failure of the engines or plane structure, extreme turbulence or pilot error or even suicide, are possible, though the use of the stolen passports has strengthened speculation of foul play.

- Without debris, there's no confirmation that the plane crashed. But finding traces of an aircraft lost at sea can take days or longer, even with a sustained search effort.

- It's not known if the two men using stolen passports had anything to do with the plane's disappearance.