North Korean hackers may attempt to cash out stolen bitcoin (BTC) worth more than $40 million, the Federal Bureau of Investigation (FBI) said in a statement on Tuesday.

The North Korea-based Lazarus Group and APT38 were reportedly behind a series of cryptocurrency hacks earlier this year, including the $60 million theft from payment processor Alphapo and the $100 million exploit of Atomic Wallet, the FBI said.

In January, the FBI named the two groups as being behind last year’s Horizon Bridge hack, which resulted in the loss of over $100 million.

Six wallets containing a total of 1,580 bitcoin ($41 million) were identified as connected to the hacker groups, and the FBI warned cryptocurrency companies against interacting with those wallets.

“The FBI will continue to expose and combat the DPRK’s use of illicit activities—including cybercrime and virtual currency theft—to generate revenue for the regime,” it said.