Thai authorities struggle to track thousands of lost or stolen passports each year. Some are known to be sold on through syndicates to drug traffickers. Others are suspected to have ended up in the hands of Islamist militants.
"Fake passports and identity fraud in general is a massive problem in Thailand," police commander and Thailand's Interpol director Apichart Suriboonya told Reuters.
Sometimes documents are sold by their owners to cover travel costs, Apichart said.
They are passed on to middlemen, Thai or foreign, who work with criminal networks, he said. The passports may be altered, for example with a new photograph, but sometimes the fraudulent user hopes to pass as the real owner.
The passenger manifest issued by Malaysia Airlines included the names of two Europeans - Austrian Christian Kozel and Italian Luigi Maraldi - who were not on the plane. Both had passports stolen on the Thai holiday island of Phuket.
The passports were used to buy tickets from travel agents in the resort town of Pattaya, to Beijing and on to Europe. Thai and foreign investigators were questioning staff at one travel agent on Monday.
There is no evidence the plane's disappearance is linked to the two passengers travelling under the stolen passports.
Police showed Reuters a copy of Maraldi's passport used to make the travel booking with what was apparently the original photograph of Maraldi in it. It was not immediately clear if the tickets were bought online or collected.
Thailand's fake document business has been flourishing for years.
In 2010, Thai and Spanish authorities arrested suspected members of an international ring providing forged passports to militants. Thai authorities say the ring may have passed fake documents to those behind the Madrid train bombings in 2004.
Pockets of Bangkok are notorious counterfeit goods emporiums with fake drivers' licences, press cards and airline cabin crew identity cards on display. The Thai capital also boasts experts in forging visas.
"Thailand is fertile territory for people looking to steal European passports, there are lots of foreigners and many foreigners visit," a European diplomat said.
The Ministry of Foreign Affairs said more than 60,000 passports - both Thai and foreign - were reported missing or stolen in Thailand between January 2012 and June 2013.
Police in Phuket said Maraldi reported his passport stolen in June last year, while Kozel's passport was reported stolen in March 2012. Police said they get reports of up to 10 lost passports a month in the province.
Phuket police officer Angkarn Yasanop said foreigners can earn $200 to sell their passport and then report it stolen. Many lost or stolen passports end up with Thais and other Southeast Asians trying to migrate for work, he said.
Larry Cunningham, who recently retired as Australia's honorary consul in Phuket, said a huge problem was tourists leaving passports as a deposit when renting jet-skis or motorbikes.
Crooked operators then make a false allegation of damage. The tourist, unwilling to pay, reports the passport stolen at an embassy on consulate and gets a new one. The old passport is sold on into the underworld.
"Phuket has some very, very unsavoury characters and they're not all Thais," Cunningham said. "Nothing would surprise me about Phuket."
Interpol's stolen and lost travel documents (SLTD) database contains 40 million records from 167 countries but its secretary general, Ronald Noble, says not enough countries are using it.
"The bad news is that, despite being incredibly cost effective and deployable to virtually anywhere in the world, only a handful of countries are systematically using SLTD to screen travellers," Noble told a conference last month.
Apichart said Thai databases were not properly linked to Interpol data.
"The technology we use in Thailand to check fraudulent identity cards is outdated at many points of entry," he said. (Additional reporting by Robert Birsel, Raju Gopalakrishnan and Jutarat Skulpichetrat; Editing by Alex Richardson)