Interpol had been looking for a disgraced finance executive for weeks when Christo Grozev, an investigative journalist, found him, hiding in Belarus. Grozev had become expert at following all but invisible digital trails — black-market cellphone data, passenger manifests, immigration records — in order to unmask Russian spies. These were the sleeper cells living in Western countries and passing as natives, or the people dispatched to hunt down dissidents around the world.

He identified the secret police agents behind one of the most high-profile assassination plots of all: the 2020 poisoning of Russian opposition leader Alexei Navalny. That revelation put Grozev in Russian President Vladimir Putin’s crosshairs. He wanted Grozev killed, and to make it happen the Kremlin turned to none othe

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