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 computers 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, a total of 48 National Health trusts were hit by Friday’s cyber-attack, of which all but six are now back to normal, according to the Home Secretary Amber Rudd.
The attack left hospitals and doctors unable to access patient data, and led to the cancellation of operations and medical appointments.
Who else has been affected by the attack?
Some reports say Russia has seen more infections than any other country. Banks, the state-owned railways and a mobile phone network were hit.
Russia’s interior ministry said 1,000 of its computers had been infected but the virus was swiftly dealt with and no sensitive data was compromised.
In Germany, the federal railway operator said electronic boards had been disrupted; people tweeted photos of a ticket machine.
France’s carmaker Renault was forced to stop production at a number of sites.