By Tom Parmenter, Sky News Correspondent
International criminal investigation teams are trying to establish how at least two passengers got on the missing Malaysia Airlines flight MH370.
It has raised serious questions for Kuala Lumpur International Airport, Malaysia Airlines and the Chinese authorities.
Officials said there were four "suspect" passengers on board, with at least two travelling on stolen passports.
Two men listed as passengers - Luigi Maraldi and Christian Kozel - have both confirmed they were not on board and that their passports were stolen in South East Asia.
Mr Maraldi's passport was stolen last summer, while Mr Kozel's was taken in Thailand in 2012.Malaysia Airlines plane vanished off the south coast of Vietnam
It means two men on board were travelling under assumed identities.
Authorities are reviewing CCTV from the airport in Kuala Lumpar to try to identify the two individuals.
Malaysian Transport Minister Hishamuddin Hussein said officials were also checking the identities of two other passengers.
"All the four names are with me," he said. "I have indicated to our intelligence agencies and I have also spoken to international intelligence agencies for assistance."
He stressed a terrorist attack was just one of the possibilities being looked at in the multi-national investigation.The circles show the two areas the search is focusing on
"We cannot jump the gun," he added. "Our focus now is to find the plane."
Director of commercial operations at Malaysia Airlines, Hugh Dunleavy, said: "As far as we're aware, every one of the people on board that aircraft had a visa to go to China, which means those passports were in the possession of the Chinese embassy before those visas were issued."
Interpol says its database currently contains 40 million records of stolen and lost travel documents from 167 countries.
Many of those will have fallen into criminal hands, but there is currently no clear link between the stolen passport holders on flight MH370 and the aircraft's disappearance.
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