Key Points

The U.S. Secret Service has seized more than $102 million in illicit digital currencies since 2015.

In a CNBC interview, the head of the agency's investigations office says cracking a crypto case is often like a "house of mirrors."

Investigators are finding thieves will transfer stolen bitcoin and other digital currencies into stablecoins.

David Smith, assistant director of investigations, said agents and analysts actively track the flow of bitcoin and other cryptocurrencies on the blockchain, similar to an old-fashioned surveillance. Best known for protecting presidents, the Secret Service also conducts financial and cybercrime investigations.