Interpol tweeted Sunday that it is "examining additional suspect #passports in connection" with the missing flight.
Earlier, the international law enforcement agency said at least two passports -- one Austrian and one Italian -- recorded in its stolen and lost travel documents database were used by passengers onboard the flight. The passports were added to the database after being stolen in separate incidents over the past two years, Interpol said.
Italy and Austria have said that none of their citizens were onboard the plane.
"No checks of the stolen Austrian and Italian passports were made by any country between the time they were entered into INTERPOL's database and the departure of flight MH 370. At this time, INTERPOL is therefore unable to determine on how many other occasions these passports were used to board flights or cross borders," the agency said in a Sunday statement.
It added that passengers were able to board planes more than a billion times last year without having their passports screened by Interpol's databases.
The Italian man whose passport was allegedly used, Luigi Maraldi, contacted the Italian consulate in Phuket, Thailand, on Saturday, after receiving a call from his parents, Italian Consul Franco Cavaliere told CNN on Sunday.
Maraldi told Reuters he was inundated with phone calls, texts and social media inquiries asking if he was alive and well. He soon discovered that he was the subject of stories about the missing plane.
Maraldi is staying on Phuket Island as a tourist, and his passport disappeared in July 2013, Cavaliere said. Maraldi told Reuters he got a new passport after his old one was stolen.
"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 Interpol Secretary General Ronald K. Noble in a statement.
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The two passengers who used the passports in question appear to have bought their tickets together.
The tickets were bought from China Southern Airlines at identical prices, paid in Thailand's baht currency, according to China's official e-ticket verification system Travelsky. The ticket numbers are contiguous, which indicates the tickets were issued together.
The two tickets booked with China Southern Airlines both start in Kuala Lumpur, fly to Beijing, and then onward to Amsterdam. The Italian passport's ticket continues to Copenhagen, the Austrian's to Frankfurt.
Authorities say they are investigating the identities of some of those onboard who appear to have issues with their passports.
"I've seen these reports about the passports. We're looking into that, but we don't have anything to confirm at this point," U.S. deputy national security adviser Tony Blinken told CNN's "State of the Union" on Sunday. "The reports certainly raise questions and concerns, and that's exactly why we're looking into them. But right now, it would be premature to speculate," he said.