During the summer of 1995, philosophy professor Linda Patrik was vacationing in Paris when she realized her brother-in-law Ted Kaczynski might be the Unabomber.

For 17 years, someone had been mailing or planting bombs across the country, killing three people and injuring 23 others. Patrik didn’t know her brother-in-law, but she knew he lived alone in a remote cabin near Lincoln, Montana.

Patrik had been reading reports about the bomber in the International Herald Tribune . His hostility toward technology reminded her of letters Ted had sent his brother David Kaczynski—who was her husband. Other details about the bomber’s life sounded familiar, too.

When David joined her in Paris, she asked whether his brother could be the man the FBI had spent years trying to apprehend. At first, David wasn’t so sure, but that would soon change.…