The hackers behind the Wannacry ransomware have tried to conceal who they are by using a virtual currency that is more anonymous than Bitcoin.
Victims paid more than $140,000 (£107,000) in bitcoins to recover files scrambled by Wannacry.
Earlier this week the gang behind the attack started to move the bitcoins out of the wallets they were paid into.
But the operators of the exchange they used to swap the bitcoins have now frozen the accounts they used.
Wannacry caught out thousands of firms around the world when it infected computers on corporate networks and encrypted their files, making them useless.
Victims were told to pay between $300 (£229) and $600 (£458) in bitcoins to have their files unscrambled and return computers to a working state.