Greek police arrested a Russian man suspected of running a money laundering operation involving turning $4 billion from illicit business activities into the digital currency bitcoin, they said on Wednesday.

Police did not name the 38-year-old, who was arrested in the northern Greek region of Chalkidiki on Monday on a U.S. warrant.

He is suspected of heading a criminal organization since 2011 that “owns, operates and manages one of the world’s leading e-crime websites”, police said in a statement.

The Russian Foreign Ministry in Moscow said it had no information on the case.

Bitcoin was the first digital currency to successfully use cryptography to keep transactions secure and hidden, making traditional financial regulation difficult if not impossible.

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