A cyber-attack that hit organisations worldwide including the UK’s National Health Service was “unprecedented”, Europe’s police agency says.
Europol also warned a “complex international investigation” was required “to identify the culprits”.
Ransomware encrypted data on at least 75,000 systems in 99 countries on Friday. Payments were demanded for access to be restored.
European countries, including Russia, were among the worst hit.
Although the spread of the malware – known as WannaCry and variants of that name – appears to have slowed, the threat is not yet over.
Europol said its cyber-crime team, EC3, was working closely with affected countries to “mitigate the threat and assist victims”.
In the UK, the head of the cyber security agency said experts were “working around the clock” to restore the systems of some 45 NHS organisations that were hit by the attack.