A bitcoin user is suing Mt. Gox over its million-dollar losses after cyberattacks bankrupted what once was the world's largest bitcoin exchange, according to Reuters.

Plaintiff Gregory Greene filed a complaint on behalf of bitcoin users in a U.S. District Court in Chicago on Thursday, accusing Mt. Gox and its CEO Mark Karpeles of negligence and fraud for not protecting the exchange from theft.

Greene, who claimed his own bitcoin holdings were about $25,000, said "Mt. Gox intentionally and knowingly failed to provide its users with the level of security protection for which they paid", Reuters reported.

The suit could be the first of many filed against Mt. Gox, which has lost an estimated $480 million dollars worth of bitcoins, a virtual currency, due to what it called a weakness in its system.

A lawyer for the embattled Japan-based company announced at a news conference on Friday that the exchange was filing for Chapter 11-style protection and stated that Mt.Gox had outstanding debts of about 6.5 billion Japanese yen ($63.6 million), the Dow Jones news agency reported.

Its customers have been unable to withdraw their bitcoins and convert them into U.S. dollars since the beginning of February. The exchange blamed the problem on a critical loophole — known as "transaction malleability" — in the cryptocurrency that it said leaves all exchanges open to hacking.

Mark Karpeles, the CEO of Mt.Gox, reiterated this belief on Friday at the news conference, blaming weaknesses in the system for the loss of its bitcoins.

The company's lawyers added that Mt.Gox may have lost nearly all of its virtual currency, leading to a black hole of 2.8 billion Japanese yen, local media reported.

(Read More: Mt.Gox CEO: 'I'm still in Japan')



The company said there were 127,000 creditors in the bankruptcy and only 0.8 percent were Japanese. Representatives added that it opted for a transparent procedure due to public outcry and will aid authorities in finding out what happened. Its liquid liabilities totaled 6.501 billion Japanese yen with its total assets being 3.842 billion Japanese yen, according to Reuters. Dow Jones added that Mt.Gox believed 750,000 of customers' coins may have been lost and 100,000 of its own, meaning a loss of around $500 million at current market prices.

(Read More: Bitcoin's Mt.Gox disappears, insolvency feared)



Mt.Gox - which once claimed it handled around 80 percent of all global dollar trades for bitcoin - is an online marketplace where people can buy or sell bitcoins using different currencies.