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In 2017, the Manhattan district attorney’s office received a tip: Unusually large amounts of cash were being withdrawn from A.T.M.s throughout the city, including a series of transactions at a special teller machine that converted cryptocurrency to cash.

Prosecutors did not quite know what they had, but worked to identify precisely who had withdrawn the money, which exceeded $1 million.

What resulted was a two-year investigation that uncovered a multimillion-dollar drug operation on the dark web, a corner of the internet people can visit anonymously with special browsers and make purchases with virtual currencies like Bitcoin and Ethereum.

In recent years, drug trafficking on the dark web has exploded, as drug traffickers and other criminals have discovered that its anonymity makes it ideal for illicit transactions using cryptocurrencies.