How do you catch a twisted genius who aspires to be the perfect, anonymous killer—who builds untraceable bombs and delivers them to random targets, who leaves false clues to throw off authorities, who lives like a recluse in the mountains of Montana and tells no one of his secret crimes?

That was the challenge facing the FBI and its investigative partners, who spent nearly two decades hunting down this ultimate lone wolf bomber.

The man that the world would eventually know as Theodore Kaczynski came to our attention in 1978 with the explosion of his first, primitive homemade bomb at a Chicago university. Over the next 17 years, he mailed or hand delivered a series of increasingly sophisticated bombs that killed three Americans and injured 24 more. Along the way, he sowed fear and panic, even threatening to blow up airliners in flight.

In 1979, an FBI-led task force that included the ATF and U.S. Postal Inspection Service was formed to investigate the “UNABOM” case, code-named for the UNiversity and Airline BOMbing targets involved. The task force would grow to more than 150 full-time investigators, analysts, and others. In search of clues, the team made every possible forensic examination of recovered bomb components and studied the lives of victims in minute detail. These efforts proved of little use in identifying the bomber, who took pains to leave no forensic evidence, building his bombs essentially from “scrap” materials available almost anywhere. And the victims, investigators later learned, were chosen randomly from library research.

We felt confident that the Unabomber had been raised in Chicago and later lived in the Salt Lake City and San Francisco areas. This turned out to be true. His occupation proved more elusive, with theories ranging from aircraft mechanic to scientist. Even the gender was not certain: although investigators believed the bomber was most likely male, they also investigated several female suspects.

The big break in the case came in 1995. The Unabomber sent us a 35,000 word essay claiming to explain his motives and views of the ills of modern society. After much debate about the wisdom of “giving in to terrorists,” FBI Director Louis Freeh and Attorney General Janet Reno approved the task force’s recommendation to publish the essay in hopes that a reader could identify the author.

After the manifesto appeared in The Washington Post and The New York Times, thousands of people suggested possible suspects. One stood out: David Kaczynski described his troubled brother Ted, who had grown up in Chicago, taught at the University of California at Berkeley (where two of the bombs had been placed), then lived for a time in Salt Lake City before settling permanently into the primitive 10’ x 14’ cabin that the brothers had constructed near Lincoln, Montana.

Most importantly, David provided letters and documents written by his brother. Our linguistic analysis determined that the author of those papers and the manifesto were almost certainly the same. When combined with facts gleaned from the bombings and Kaczynski’s life, that analysis provided the basis for a search warrant.