Greek police have arrested one of the central figures in the Bitcoin exchange BTC-e on suspicion of money laundering. Russian citizen Alexander Vinnik was arrested in Greece at the request of US law enforcement, according to a Reuters report. The report cites two sources close to BTC-e who describe Vinnik as “a key person behind the platform.” The exchange itself has been down since Tuesday evening.

The BTC-e exchange has long been a favorite of criminals, as its headquarters in Russia places it outside the reach of US and European law enforcement. Recent Google research found that 95 percent of ransomware cash-outs occurred through the BTC-e exchange, although it’s unclear whether the exchange itself would be liable for those payments. Those payments have made BTC-e one of the largest bitcoin exchanges, regularly handling more than 3 percent of total Bitcoin transactions.

“Our chief suspect for involvement in the Mt Gox theft”

The indictment against Vinnik focuses on money laundering and other financial crimes, but mentions BTC-e was connected to proceeds from the CryptoWall ransomware and the Mt Gox theft, many of them passing through accounts directly connected to Vinnik.

“Cryptocurrencies such as Bitcoin provide people around the world new and innovative ways of engaging in legitimate commerce,” said U.S. Attorney Colin Stretch, whose office will be prosecuting the case. “As this case demonstrates, however, just as new computer technologies continue to change the way we engage each other and experience the world, so too will criminals subvert these new technologies to serve their own nefarious purposes.”

Analysts have long believed that Vinnik is connected the massive theft that brought down the Mt Gox bitcoin exchange in 2014. In a report released shortly after the arrest, the security firm Wizsec described Vinnik as “our chief suspect for involvement in the Mt Gox theft (or the laundering of the proceeds thereof).” Analyzing transactions on the blockchain, the group claims to have seen the proceeds from several high-profile Bitcoin thefts pass through wallets listed under Vinnik’s name.

Mt Gox was the most popular exchange of early Bitcoin users, until it was revealed to be catastrophically insolvent as a result of a long-running theft, which made off with as much as $400 million in bitcoin. The culprit has never been identified, and has remained a subject of intense speculation throughout the bitcoin community.

Previous analysis of the theft indicated the crucial compromise occurred as early as 2011, with subsequent withdrawals going unnoticed until years afterward. BTC-e was founded in July 2011.

Sarah Jeong contributed additional reporting to this article.

Update 7/27 9:23 AM ET: Updated with affidavit and Department of Justice statement.