The South Korean National Intelligence Service (NIS) believe that North Korean hackers are responsible for a series of attacks on a popular bitcoin exchange, Bithumb. The BBC reports that the NIS is planning to pass the evidence it has gathered to prosecutors. A Bithumb employee’s computer was targeted in the attack, which dates back to February 2017.

During this time period, around 7.6 billion won ($6.99m) of various cryptocurrencies were stolen from the exchange, along with the personal details of 30,000 customers. Reports of the attack first came to light in July, when the hackers demanded a $5.5m ransom in exchange for the deletion of the stolen details. In September, it is believed that North Korean hackers targeted the Coinis exchange.

South Koreans were targeted again in October, with the South Korean National Police Agency confirming that 25 employees from four exchanges were attacked by hackers in an attempt to gain access to accounts or sensitive information.

Tensions between the two countries remain high as a result of ballistic missile tests carried out by North Korea, and it is believed that the cryptocurrency attacks are an attempt to avoid international sanctions which prevent money from entering or leaving the country.