The international law enforcement agency Interpol confirmed Sunday that at least two passengers on the missing Malaysia Airlines flight used stolen passports that were registered as missing with them and said it was "examining additional suspect passports."
Interpol also said no checks of its database had been made by any country on an Austrian and an Italian passport between the time that they were stolen, months ago, and the departure of the flight.
On Saturday, officials began investigating terrorism concerns on Flight MH370 after authorities in Vienna and Rome confirmed that two passengers listed on the flight manifest were not actually on the flight and recently had their passports stolen.
The passports went missing in Thailand in 2012 and 2013, according to Interpol.
Because no checks were ever made, Interpol said it was unable to determine on how many other occasions the passports were used to board flights or cross borders.
"Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol's databases," said the organization's secretary general, Ronald K. Noble.
Interpol is investigating the true identities of the passengers who were aboard the plane that broke contact and disappeared two hours after it took off from Kuala Lumpur, Noble said in a statement.
Interpol is also checking each passport on Flight MH370 to determine whether any other identification documents had been reported stolen, it said.