Cybersecurity researchers named the worm WannaCry, after the .wncry extension it added to file names after encrypting them. As it paralyzed machines and demanded its bitcoin ransom, WannaCry was jumping from one machine to the next using a powerful piece of code called EternalBlue, which had been stolen from the National Security Agency by a group of hackers known as the Shadow Brokers and leaked onto the open internet a month earlier. It instantly allowed a hacker to penetrate and run hostile code on any unpatched Windows computer—a set of potential targets that likely numbered in the millions. And now that the NSA’s highly sophisticated spy tool had been weaponized, it seemed bound to create a global ransomware pandemic within hours.

“It was the cyber equivalent of watching the moments before a car crash,” says one cybersecurity analyst who worked for British Telecom at the time and was tasked with incident response for the NHS. “We knew that, in terms of the impact on people’s lives, this was going to be like nothing we had ever seen before.”

As the worm spread around the world, it infected the German railway firm Deutsche Bahn, Sberbank in Russia, automakers Renault, Nissan, and Honda, universities in China, police departments in India, the Spanish telecom firm Telefónica, FedEx, and Boeing. In the space of an afternoon, it destroyed, by some estimates, nearly a quarter-million computers’ data, inflicting between $4 billion and $8 billion in damage.

For those watching WannaCry’s proliferation, it seemed there was still more pain to come. Josh Corman, at the time a cybersecurity-focused fellow for the Atlantic Council, remembers joining a call on the afternoon of May 12 with representatives from the US Department of Homeland Security, the Department of Health and Human Services, the pharmaceutical firm Merck, and executives from American hospitals. The group, known as the Healthcare Cybersecurity Industry Taskforce, had just finished an analysis that detailed a serious lack of IT security personnel in American hospitals. Now WannaCry seemed poised to spread to the US health care system, and Corman feared the results would be far worse than they had been for the NHS. “If this happens en masse, how many people die?” he remembers thinking. “Our worst nightmare seemed to be coming true.”

At around 2:30 on that Friday afternoon, Marcus Hutchins returned from picking up lunch at his local fish-and-chips shop in Ilfracombe, sat down in front of his computer, and discovered that the internet was on fire. “I picked a hell of a fucking week to take off work,” Hutchins wrote on Twitter.

Within minutes, a hacker friend who went by the name Kafeine sent Hutchins a copy of WannaCry’s code, and Hutchins began trying to dissect it, with his lunch still sitting in front of him. First, he spun up a simulated computer on a server that he ran in his bedroom, complete with fake files for the ransomware to encrypt, and ran the program in that quarantined test environment. He immediately noticed that before encrypting the decoy files, the malware sent out a query to a certain, very random-looking web address:

That struck Hutchins as significant, if not unusual: When a piece of malware pinged back to this sort of domain, that usually meant it was communicating with a command-and-control server somewhere that might be giving the infected computer instructions. Hutchins copied that long website string into his web browser and found, to his surprise, that no such site existed.