They demanded that the facility's authorities pay a ransom by July 18, Serbian website Danas reported on Monday.
The Zagreb teaching hospital was hacked last Thursday by the Russian LockBit 3.0 virus, used to steal data that enables hackers to demand a ransom.
Croatian investigators and cybersecurity company HackManac confirmed that a Russian group was responsible.
The hackers said they gained access to medical records, organ and donor data, employee data, and contracts signed with external companies.
The amount of the ransom demanded from the hospital was not disclosed.
The U.S. Justice Department estimates that the LockBit virus has so far been used to attack more than 2,500 entities, extorting ransoms worth more than $500 million.
The day before the Croatian hospital was targeted, there was a hacking attack on state institutions, including the finance ministry and the central bank.
Two days later, Russians from the Noname057(16) group claimed responsibility, arguing that they were targeting “Russophobic countries.” The group has previously attacked entities in Poland and Spain.
