Interpol says it's greatly concerned that passengers could get on the missing Malaysian Airlines plane with stolen passports.

Two are known to have boarded with false identities.

Another two are under investigation, and terrorism is one of the lines of inquiry.

An international search is under way for Flight 370, which was carrying 239 people when it disappeared on Friday.

Italian Luigi Maraldi lost his passport, and it was used to get onto the flight.

He didn't think it could be used after he reported it lost.