Two hacker groups associated to North Korea have been held responsible for the $620 million Axie Infinity heist last month, the Federal Bureau of Investigation (FBI) said Thursday. 

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Axie Infinity is a non-fungible token-based online video game developed by Vietnamese studio Sky Mavis, which uses Ethereum-based cryptocurrencies for in-game transactions. Last Month, Ronin –  an Ethereum-linked sidechain made specifically for Axie Infinity was hacked for over 173,600 Ether (ETH) and 25.5 million USD Coin (USDC) for a combined value of close to $620 million.

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The hack, one of the biggest to hit cryptocurrencies, had raised questions about security of the digital currency. 

“Through our investigations we were able to confirm Lazarus Group and APT38, cyber actors associated with (North Korea), are responsible for the theft,” the FBI said in a statement.

While not much is known about the Lazarus Group, researchers say that it has been active since 2010. The North Korean group was also ccused of hacking into Sony Pictures Entertainment after a satirical film. ‘The Interview’, mocked Kim Jong Un.

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Blockchain data platform Chainalysis in January reported that North Korean hackers stole around $400-million worth of cryptocurrency through cyber attacks last year. 

Chainalysis, a blockchain analytics firm, on Twitter said that the  United States Department of Treasury’s Office of Foreign Assets Control updated its Specially Designated Nationals and Blocked Persons’ List with an Ethereum address attributed to the Lazarus Group. The Ronin Network later confirmed that the Federal Bureau of Investigation had attributed the Ronin validator security breach to the Lazarus Group.

“North Korea has been unique in that they have APT groups focused on stealing cryptocurrency. As North Korea is highly-sanctioned, cryptocurrency thefts are also a national security interest for them. Sanctioning the wallet probably won’t help too much as there are exchanges that don’t respect the OFAC list,” ” John Bambenek, principal threat hunter at digital IT and security operations company Netenrich Inc., told SiliconANGLE.