On August 19, a twentysomething man who goes by the online handle ZachXBT was walking into an airport to board a flight home—which airport, his real name, where home is, he’d rather not say—when he saw an alert on his phone. A sum of bitcoins had just been transferred to a small cryptocurrency exchange, one of many whose transactions he constantly monitors on Bitcoin’s blockchain for signs of criminal money laundering. The alert piqued his interest: This transaction was worth around $600,000, a cash-out of funds that was easily 10 times bigger than the typical trade on that service.
When he reached his gate, another ping alerted him to a
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