The Tokyo bitcoin exchange that filed for bankruptcy protection blamed theft through hacking for its losses and said it was looking into a criminal complaint.
Mt. Gox's chief Mark Karpeles posted the announcement on the company website Monday, outlining the events that resulted in insolvency and saying there was "high probability" theft was behind the disappearance of bitcoins.
He said a huge number of transactions must be investigated, but efforts are underway to bring Mt. Gox back into business to repay its debts.
Karpeles has said 750,000 bitcoins from users and another 100,000 belonging to the company disappeared.
Mt. Gox was unplugged last week as rumors of its insolvency swirled, adding to fears about bitcoins overall.
Karpeles said illegal access in early February abused a bug in the computer system.