Following a two-month investigation, officers led by immigration bureau chief Ronnasin Phusara arrested a Thai woman nicknamed "Mama", who allegedly sold the visa stickers stolen from the Kuala Lumpur consulate to foreigners.
Investigators are taking the case seriously as they fear fake visas may be used by criminals posing a risk to national security, such as terrorists and drug traffickers.
Pol Maj Gen Ronnasin's team began its investigation after the immigration police in Mukdahan arrested a Cameroon national as he crossed into Thailand from Laos on July 20.
Samuel Ncha Ekwale possessed a visa supposedly issued by the Thai consulate in Kuala Lumpur, but authorities were suspicious about how he had obtained it as there was no entry or exit stamp from Malaysia in his passport.
The suspect allegedly confirmed he had not travelled to Malaysia and admitted he obtained the visa from "Mama", who had offered to help him get a Thai visa without going through the usual lengthy procedure. He paid about 30,000 baht for the service.
Pol Maj Gen Ronnasin's investigators then began to search for Mama.
They tracked Mr Ekwale's payment and found he had transferred the money to a bank account belonging to a Nigerian man he believed was Mama's husband.
With this information, officers were able to identify Mama as Piyamat Bandasak, 39, who is a former Spanish interpreter for the Colombian consulate in Thailand.
The investigators then asked the court to issue a warrant for Ms Piyamat's arrest.
While waiting for the warrant, the Foreign Ministry announced in early August that 300 visa stickers had disappeared from the Thai consulate in the Malaysian capital.
Immigration police across the country were told to watch out for foreigners arriving on visas issued by Kuala Lumpur.
The ministry also reported similar cases in other countries.
About 500 visa stickers disappeared from the Thai consulate in Laos' Savannakhet province and 2,000 visa stickers had vanished from the Thai consulate in The Hague in the Netherlands.
As ministry officials set about investigating the lost stickers case, Pol Maj Gen Ronnasin's team obtained an arrest warrant for Ms Piyamat and apprehended her at her home in Bangkok's Bung Kum district on Wednesday last week.
The immigration bureau's deputy chief of investigations, Chartchai Iamsaeng, said the probe identified the suspected mastermind behind the missing Kuala Lumpur visa stickers as Indian national Kumar Ramesh, 41.
Mr Ramesh is known to be in a relationship with a woman who works at the consulate. They are suspected of working together in the visa racket.
The investigation revealed the woman allegedly stole the visa stickers from the consulate and sold them to others, including Ms Piyamat.
There are two types of customers who buy the stolen visa stickers, Pol Col Chartchai said.
The first group comprises those who only want to bypass long procedures in applying for visas.
The other group consists of citizens of countries on the Foreign Affairs Ministry's immigration watch list.
Investigators found 259 of the visas stolen from Kuala Lumpur have been used. Among those foreigners known to have used the fake visas, 77 are still in Thailand. Officers have arrested 23 of them.
Pol Col Chartchai and other investigators are continuing their search for the remaining 54 fake visa-buyers who are still in the country.
Among the foreigners using the visas are nationals from Nigeria, Iran and Pakistan, which are among the countries on the Foreign Ministry's watch list.
"Considering their nationalities, I believe many of them came to Thailand with ill-intentioned purposes," Pol Col Chartchai said.
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