Mark Karpelès, the head of what once was the world's biggest bitcoin exchange, looked calm and restrained, with even a hint of a smile Friday evening when he appeared at a hastily organized news conference to announce that his company filed for bankruptcy protection.

It wasn't the picture one would expect from a man who said he may have lost all of his company's investors' virtual coins worth millions of dollars, stoking global concern for the future of the digital currency.

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