Everyone knows there’s no such thing as poisoned Halloween candy. No chemically laced chewing gum. No razor blades hidden in caramel apples. Researchers have spent years debunking this urban legend, which, they say, was born from a certain strain of cultural anxiety—fear of strangers, of uncertainty, of too much sugar. Almost certainly, the cruelest thing a neighbor will do to your trick-or-treating kid is hand them a toothbrush.

But about 30 years ago, across much of Japan, this irrational, deep-seated fear actually came true. Over the course of a year and a half, a cryptic group blackmailed the country’s biggest candy companies. They filled supermarket shelves with cyanide-laced chocolates. They wrote elaborate, teasing letters detailing their exploits, which were published in national newspapers. They consistently foiled the nation’s police force. And to this day, no one has any idea who they were.