Europol, the European Union’s police agency, said the onslaught was at “an unprecedented level and will require a complex international investigation to identify the culprits.”

In Russia, government agencies insisted that all attacks had been resolved. Russia’s health ministry said its attacks were “effectively repelled.”

The ransomware appeared to exploit a vulnerability in Microsoft Windows that was purportedly identified by the U.S. National Security Agency for its own intelligence-gathering purposes. Microsoft had made fixes for older systems, such as 2001’s Windows XP, available only to mostly larger organizations, including Britain’s National Health Service, that paid extra for extended technical support. Britain's National Cyber Security Centre and its National Crime Agency were looking into the UK incidents, which disrupted care at National Health Service facilities, forcing ambulances to divert and hospitals to postpone operations. Pictures posted on social media showed screens of NHS computers with images demanding payment of $300 worth of the online currency Bitcoin, saying: “Ooops, your files have been encrypted!”

NHS Digital said the attack “was not specifically targeted at the NHS and is affecting organizations from across a range of sectors.” It said 16 NHS organizations had reported being hit. Indeed, while FedEx Corp. reported that its Windows computers were “experiencing interference” from malware — it wouldn’t say if it had been hit by the ransomware — other impacts in the U.S. were not readily apparent on Saturday. In Spain, some big firms took pre-emptive steps to thwart ransomware attacks following a warning from Spain's National Cryptology Centre of "a massive ransomware attack".
A large scale international cyber-attack using the WannaCry ransomware package disrupts computer and telephone systems in thousands of cases across 99 countries. Affected organisations include: England's NHS, with doctors reporting messages demanding money; MegaFon and Sberbank in Russia; Telefónica, Iberdrola, and Gas Natural in Spain; and FedEx.