It is believed the passports were stolen and taken out of Thailand without being used at immigration checkpoints, Pol Maj Gen Warawut Taweechaikarn said.
The officer said criminals faced difficulties producing fake passports due to sophisticated anti-counterfeiting techniques, so resorted to buying real passports from gangs of thieves, which targeted foreign tourists in Thailand.
The gangs would replace the passport holders' photos and alter certain information in the documents before using them, he added.
Fears that terrorism may be linked to the disappearance of Malaysia Airlines Flight MH370 arose after Austria and Italy said passports used by two male passengers were stolen from their citizens. Malaysia said it had identified one of the illegal-passport holders.
“We are trying to ascertain if the two holders of false passports entered Malaysia, legally or illegally,” police inspector-general Khalid Abu Bakar said on Monday.
Closed-circuit television footage exists of the two people who used the false passports, said director general of the Department of Civil Aviation Azharuddin Abdul Rahman. “We have the CCTV recordings of those passengers from check-in right through to the departure point,” he said.
Interpol said in a statement that at least two passports recorded in its database, one Austrian and one Italian, were used by passengers on the flight after being reported stolen in Thailand. Two people using Italian and Austrian passports on the flight, Luigi Maraldi, 37, and Christian Kozel, 30, had consecutive ticket numbers, according to the Chinese e-ticket verification system Travelsky.
Both tickets were issued on March 6, according to the website of China Southern Airlines Co, which was a code-share on the flight.