North Korean cybercriminals used “sophisticated” social engineering techniques to steal some $66 million in two cryptocurrency heists earlier this year, according to recently released reports.

The decentralized finance platform Radiant Capital said on its blog on Friday that the DPRK threat actor Citrine Sleet, also known as UNC4736 and AppleJeus, carried out a “highly sophisticated cyberattack” against its systems and stole $50 million in cryptocurrency.

U.S. cybersecurity firm Mandiant attributed the attack to the North Korean cybercrime group with “high confidence” after being contracted by Radiant to investigate the theft, the blog post added.