A global cyberattack leveraging hacking tools widely believed by researchers to have been developed by the U.S. National Security Agency hit international shipper FedEx, disrupted Britain's health system and infected computers in nearly 100 countries on Friday.

Extortionists tricked victims into opening malicious malware attachments to spam emails that appeared to contain invoices, job offers, security warnings and other legitimate files.

The ransomware encrypted data on the computers, demanding payments of $300 to $600 to restore access. Security researchers said they observed some victims paying via the digital currency bitcoin, though they did not know what percent had given in to the extortionists.