Excerpts from wiki:
> Before the publication of Industrial Society and Its Future, Kaczynski's brother, David, was encouraged by his wife to follow up on suspicions that Ted was the Unabomber.[91] David was dismissive at first, but he took the likelihood more seriously after reading the manifesto a week after it was published in September 1995. He searched through old family papers and found letters dating to the 1970s that Ted had sent to newspapers to protest the abuses of technology using phrasing similar to that in the manifesto.[92]
> In early 1996, an investigator working with Bisceglie contacted former FBI hostage negotiator and criminal profiler Clinton R. Van Zandt. Bisceglie asked him to compare the manifesto to typewritten copies of handwritten letters David had received from his brother. Van Zandt's initial analysis determined that there was better than a 60 percent chance that the same person had written the manifesto, which had been in public circulation for half a year. Van Zandt's second analytical team determined a higher likelihood. He recommended Bisceglie's client contact the FBI immediately.[96]
> In February 1996, Bisceglie gave a copy of the 1971 essay written by Ted Kaczynski to Molly Flynn at the FBI.[87] She forwarded the essay to the San Francisco-based task force. FBI profiler James R. Fitzgerald[98][99] recognized similarities in the writings using linguistic analysis and determined that the author of the essays and the manifesto was almost certainly the same person. Combined with facts gleaned from the bombings and Kaczynski's life, the analysis provided the basis for an affidavit signed by Terry Turchie, the head of the entire investigation, in support of the application for a search warrant.[87]