In September 1982, seven people in the Chicago suburbs died after taking Extra-Strength Tylenol that had been laced with potassium cyanide, and whoever did it was never caught. The case rewrote American consumer safety law, killed the two-piece capsule, and gave us every foil seal and tamper-evident collar on every bottle of medicine you've ever opened. More than forty years later, the identity of the killer remains one of the most frustrating unsolved mysteries in American criminal history.