EARLY ONE FALL morning in 2017, in a middle-class suburb on the outskirts of Atlanta, Chris Janczewski stood alone inside the doorway of a home he had not been invited to enter. Moments earlier, armed Homeland Security Investigations agents in ballistic vests had taken up positions around the tidy two-story brick house, banged on the front door, and when a member of the family living there opened it, swarmed inside. Janczewski, an Internal Revenue Service criminal investigator, followed quietly behind. Now he found himself in the entryway, in the eye of a storm of activity, watching the agents search the premises and seize electronic devices.

They separated the family, putting the father, an assistant principal at the local high school and the target of their investigation, in one room; his wife in another; the two kids into a third. An agent switched on a TV and put on Mickey Mouse Clubhouse in an attempt to distract the children from the invasion of their home and the interrogation of their parents.

Janczewski had come along on this raid only as an observer, a visitor flown in from Washington, DC, to watch and advise the local Homeland Security team as it executed its warrant. But it had been Janczewski’s investigation that brought the agents here, to this average-looking house with its well-kept yard among all the average-looking houses they could have been searching, anywhere in America. He had led them there based on a strange, nascent form of evidence. Janczewski had followed the links of Bitcoin’s blockchain, pulling on that chain until it connected this ordinary home to an extraordinarily cruel place on the internet—and then connected that place to hundreds more men around the world. All complicit in the same massive network of unspeakable abuse. All now on Janczewski’s long list of targets.

Over the previous few years, Janczewski, his partner Tigran Gambaryan, and a small group of investigators at a growing roster of three-letter American agencies had used this newfound technique, tracing a cryptocurrency that once seemed untraceable, to crack one criminal case after another on an unprecedented, epic scale. But those methods had never led them to a case quite like this one, in which the fate of so many people, victims and perpetrators alike, seemed to hang on the findings of this novel form of forensics. That morning’s search in the suburb near Atlanta was the first moment when those stakes became real for Janczewski. It was, as he would later put it, “a proof of concept.”

From where Janczewski was positioned at the front of the house, he could hear the Homeland Security agents speaking to the father, who responded in a broken, resigned voice. In another room, he overheard the agents questioning the man’s wife; she was answering that, yes, she’d found certain images on her husband’s computer, but he’d told her he had downloaded them by accident when he was pirating music. And in the third room he could hear the two grade-school-age children—kids about as old as Janczewski’s own—watching TV. They asked for a snack, seemingly oblivious to the tragedy unfolding for their family.

Janczewski remembers the gravity of the moment hitting him: This was a high school administrator, a husband and a father of two. Whether he was guilty or innocent, the accusations this team of law enforcement agents were leveling against him—their mere presence in his home—would almost certainly ruin his life.

Janczewski thought again of the investigative method that had brought them there like a digital divining rod, revealing a hidden layer of illicit connections underlying the visible world. He hoped, not for the last time, that it hadn’t led him astray.

ON A SUMMER’S day in London a few months earlier, a South Africa-born tech entrepreneur named Jonathan Levin had walked into the unassuming brick headquarters of the UK’s National Crime Agency—Britain’s equivalent to the FBI—on the south bank of the Thames. A friendly agent led him to the building’s second floor and through the office kitchen, offering him a cup of tea. Levin accepted, as he always did on visits to the NCA, leaving the tea bag in.

The two men sat, cups in hand, at the agent’s desk in a collection of cubicles. Levin was there on a routine customer visit, to learn how the agent and his colleagues were using the software built by the company he’d cofounded. That company, Chainalysis, was the world’s first tech firm to focus solely on a task that a few years earlier might have sounded like an oxymoron: tracing cryptocurrency. The NCA was one of dozens of law enforcement agencies around the world that had learned to use Chainalysis’ software to turn the digital underworld’s preferred means of exchange into its Achilles’ heel.

When Bitcoin first appeared in 2008, one fundamental promise of the cryptocurrency was that it revealed only which coins reside at which Bitcoin addresses—long, unique strings of letters and numbers—without any identifying information about those coins’ owners. This layer of obfuscation created the impression among many early adherents that Bitcoin might be the fully anonymous internet cash long awaited by libertarian cypherpunks and crypto-anarchists: a new financial netherworld where digital briefcases full of unmarked bills could change hands across the globe in an instant.