A UK company has been linked to the laundering of 650,000 stolen bitcoins worth £4.5bn, a BBC Radio 4 investigation has found.
The coins were taken by hackers from Tokyo-based Bitcoin exchange Mt Gox, leaving tens of thousands of customers out of pocket.
It's not clear who is in control of the London-based firm Always Efficient LLP.
Mt Gox operator Mark Karpeles apologised to investors and said he was co-operating with the investigation.
The FBI has charged a Russian national with laundering the stolen bitcoins.
Mt Gox matched up those who wanted to buy the crypto-currency with dollars, pounds and other international denominations with those wanting to sell bitcoins, and handled an estimated 70% of the world's Bitcoin trade.
The exchange was originally set up to trade cards from a game set in a world of wizards, spells and monsters. When it turned its focus to crypto-currencies, it appeared to be a huge success story.
Almost half of Bitcoin trading is done in Japanese yen, and there's even a Japanese girl group, the Virtual Currency Girls, which reflects Japan's growing craze for virtual money.
But a group of amateur investigators, WizSec, found that hackers had targeted Mt Gox.
They had systematically pilfered users' accounts, hiding their tracks from Mt Gox operators for years.
The FBI says BTC-e was a hub for cyber-crime and helped to launder money from hacks, including ransomware attacks of the kind that hit the NHS and other organisations last year.
But trying to find out who operates BTC-e isn't easy.The exchange claimed to be operated by a British company called Always Efficient LLP.
Under Japanese bankruptcy law, the remaining 200,000 Mt Gox bitcoins are valued at £300 each, the price they held when the exchange collapsed in 2014.