A Russian national has been charged by a US grand jury over the alleged laundering of billions of dollars using virtual currency Bitcoin.
Alexander Vinnik, 38, was arrested in Greece near the northern city of Thessaloniki on a US warrant.
A US attorney later announced he had been charged with 21 counts, including laundering stolen funds from the hack of the "Mt Gox" Bitcoin exchange.
The gang is believed to have laundered some $4bn (£3bn) over several years.
Mr Vinnik is said to be the "brain" behind the illegal operations. He has so far made no public comments.
The US Attorney's Office in California's northern district said the indictment covered "operating an unlicensed money service business, money laundering and related crimes."
In particular, it alleged that Mr Vinnik received funds "from the infamous computer intrusion or 'hack' of Mt Gox."
Mt Gox was a Bitcoin exchange - a place where real money could be exchanged for the digital currency - which was once the largest of its kind.
It was forced to declare bankruptcy in 2014 after a hack which stole about 850,000 Bitcoins - worth millions at the time.
Local media reports say that Greece will soon begin negotiations with the US about Mr Vinnik's extradition.
If convicted in the US, Mr Vinnik could face up to 20 years in prison on the money laundering charges.
Bitcoin is a digital currency that operates completely online.
Each Bitcoin is basically a computer file which is then stored in a "digital wallet" app on a smartphone or computer.
Unlike traditional currencies such as the US dollar, Bitcoin has no central bank and is not backed by any government.