Cryptocurrency marketplace NiceHash said the contents of its bitcoin wallet had been stolen in a security breach and one executive said nearly $64 million had been lost.

NiceHash head of marketing Andrej P. Škraba told Reuters on Thursday that the hack was “a highly professional attack with sophisticated social engineering” and that about 4,700 bitcoin, worth about $63.92 million at current prices, were lost.

He said the company was co-operating with local authorities but declined to give more information.

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