Jihan Wu-led Chinese cryptocurrency mining hardware manufacturing giant Bitmain is suing a mysterious hacker – referred to as “John Doe” in court documents – for allegedly stealing $5.5 million worth of cryptocurrency.

The lawsuit is currently under review in the U.S. District Court for the Western District of Washington, Seattle. Court documents reveal that the accused cyber-criminal was able to steal an amount that “exceeds” $5.5 million in “bitcoin and other digital assets,” including some 617 BTC that Bitmain was holding. The intrusion reportedly happened back in April, which saw the hackers access Bitmain’s account at leading cryptocurrency exchanges Binance and Bittrex.

The hacker, according to the case documents, allegedly used the stolen Bitcoin to artificially inflate – or pump – the price of Decentraland. Bitmain also asserts that the unnamed hacker funneled Bitcoin into Ethereum, then back into Decentraland. Bitmain says the Bitcoins were sold at “far above the going market rate.”

Bitmain, who released new miners last week, claims that the hacker made a number of trades back and forth between Bitcoin and Decentraland on Binance, then sent the cryptocurrency “ultimately into a digital wallet on the Bittrex cryptocurrency trading platform.”

The hacker is only referred to as “John Doe” as Bitmain has yet to identify the cyber-criminal suspected of stealing their cryptocurrency. The lawsuit is in place so Bitmain can “hold them accountable, and recover its stolen property.”

How a hacker was able to gain access to Bitmain’s Binance accounts is anyone’s guess. Given Binance’s reputation for some of the best security standards across the industry, the hacker would need to be highly sophisticated, or be someone close to the company’s operations – specifically someone with access to the firm’s cryptocurrency holdings. Binance uses two-factor authentication, among other security measures, to prevent unauthorized access to user accounts.

Even Bitmain needs to heed the cryptocurrency community’s advice: it’s always best to store funds in a cold storage solution rather than on a cryptocurrency exchange.