CodeMonkeyZ, [08.06.21 08:09]
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FBI Affidavit explaining how they ended up recovering ransomware bitcoins was released.
The FBI watched the suspects try to launder the bitcoin through different addresses until the bitcoin landed on an exchange (the redacted section is the name of the exchange that the hackers used).
From there, the FBI simply contacted the exchange and asked for the private key to the coins.

https://storage.courtlistener.com/recap/gov.uscourts.cand.379840/gov.uscourts.cand.379840.1.0.pdf