Two men had boarded MH370 with stolen passports.
Another two, who travelled on European passports, possibly Ukrainian, are being investigated by the Malaysian authorities, Federal Bureau of Investigation and Interpol.
Five others failed to board the flight and had their baggage offloaded, although no further information has emerged on their identities.
All of these have raised suspicions that the Beijing-bound Boeing 777-200 jet, which vanished on Saturday with 239 people on board, may have been hijacked or bombed, though Malaysia's government stressed it was considering all possibilities.
The head of Malaysia's civil aviation authority told reporters yesterday that two "imposters" with fake passports had been identified by investigators as they made their way from check-in, through immigration to the departure gate.
'IMPOSTERS' HAD TRAVELLED TOGETHER
It has emerged that the two men, who took on the identity of Christian Kozel, an Austrian, and Luigi Maraldi, an Italian, had purchased their air tickets in the Thai beach resort of Pattaya.
The e-ticket numbers for their flights are consecutive and both were paid for in Thai baht.
"Kozel" was booked to travel from Kuala Lumpur to Beijing on the Malaysia Airlines Boeing 777, then on to Amsterdam and Frankfurt.
"Maraldi" was booked on the same flights until Amsterdam, where he was to continue to Copenhagen.
"While 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 Skype interview with CNN, former Inspector General of the US Department of Transportation Aviation Attorney Mary Schiavo added: "It does raise red flags immediately (that the two men boarded the plane on forged passports), particularly given the destination of the flight to Beijing.
"It's rare that you have one stolen passport, much less two stolen passports on a flight. It's starting to look like more than a coincidence."
CNN law enforcement analyst Tom Fuentes added: "You wonder who was using it? What were their motives?
"Were they using it to check luggage in that matched the tickets, and maybe the luggage contained explosives? So, it's a great concern when people use false documents to board international aircraft."
The timing of the incident, a week after knife-wielding assailants killed at least 29 people at a train station in the south-western Chinese city of Kunming, led to speculation that militants from China's Uighur Muslim minority could be involved.
One of the Malaysian officials said the authorities were not ruling out Uighur involvement in the jet's disappearance, noting that Uighurs were deported to China from Malaysia in 2011 and 2012 for carrying false passports.
"People with fake passports present a huge problem for security," said security expert Yang Shu at China's Lanzhou University. "I strongly believe that they had something to do with the plane going missing."
Get The New Paper for more stories.