A US data analysis firm says hackers affiliated with North Korea are believed to be responsible for a massive loss of assets from Japanese cryptocurrency exchange DMM Bitcoin in May.

Chainalysis says it traced the flow of funds following the security breach that resulted in the loss of bitcoin worth 305 million US dollars at the time.

The result shows that the hackers used a service by which stolen bitcoin are mixed with those of other users to make the currency harder to track. The attackers are said to have then laundered the funds such as by moving them to a foreign online marketplace.

Chainalysis says this method of money laundering is typically employed by North Korea, and that some of the intermediary addresses used in the transactions matched those previously linked to Pyongyang.

The firm says the case highlights the importance of online security. It adds that it is vital to keep in touch with foreign law enforcement authorities and promote international cooperation to tackle such crimes.

DMM Bitcoin has refrained from commenting on the findings, citing an ongoing police investigation in Japan. The firm announced this month that it will shut down its business after handing over customer accounts and assets to another crypto exchange.

Chainalysis says that in 2024 North Korea-affiliated hackers stole crypto assets worth 1.3 billion dollars. That includes the alleged theft from DMM Bitcoin.