Two South Koreans arrested in Japan last fall on suspicion of violating the country's immigration law had been involved in an operation to obtain foreign currency under the direction of a man believed to be a North Korean agent, investigative sources said Thursday.

Japanese police have classified their cooperation as a spy case involving North Korea and believe it is just one part of a foreign currency acquisition network put in place by Pyongyang to circumvent economic sanctions.

According to the sources, the man who gave the orders to the South Korean pair via several other individuals mainly operated from China, using a fictitious name such as Ri Ho Nam.

He is reported to have belonged to North Korea's primary intelligence agency called the Reconnaissance General Bureau and is believed to still be a central figure in the foreign currency acquisition network.

The man is also said to be the model for a high-ranking North Korean official who appears in "The Spy Gone North," a film loosely based on the memoirs of a former South Korean agent.

The two South Koreans -- a man in his 60s and a woman in her 70s -- established a trading company at the end of 2016 in a condominium in Tokyo to import and sell nutritional drinks from their home country.

They were arrested in October and November last year by Tokyo police for allegedly conducting activities outside of their status of residence and entering Japan on fraudulently obtained documents.

An analysis of seized materials revealed that the pair used their company to trade marine products with North Korea and were involved in a plan to build a liquefied petroleum gas terminal near the North Korean border in Russia's Far East.

In what has been deemed as an operation led by North Korea's spy agency to acquire foreign currency, the police believe some of the funds obtained by the man and the woman through their business and other means were routed to the man who had instructed them.

The South Korean man, who had been living in Japan for a long time, and the woman, who had repeatedly entered the country on short-stay visas since around November 2017, were collaborators in the operation, according to the sources. But Japanese prosecutors have decided not to indict them and the pair are no longer in custody.

South Korea's Yonhap News Agency reported in February that the man who had directed the pair met with an official of Korea Gas Corp. at a hotel in Vladivostok in Russia's Far East in late 2019. At that time, he asked if the South Korean gas company would buy Russian-produced gas purchased by North Korea, but the firm had declined.