In an interview with "20/20 on ID Presents: Homicide" on Investigation Discovery, Kaczynski's sister-in-law, Linda Patrik, who was one of the first to identify Kaczynski as the Unabomber, recalled the first time she suspected Kaczynski was responsible for the serial bombings.
Between 1978 and 1995, Kaczynski placed or mailed 16 bombs that killed three people and injured 23 others, according to authorities.
In 1995, before he was identified as the Unabomber, he demanded newspapers to publish a long manuscript he had written, saying the killings would continue otherwise. Both the New York Times and Washington Post published the 35,000-word manifesto later that year at the recommendation of the Attorney General and the Director of the FBI.
A professor of philosophy, Patrik recognized familiar sounding ideas in the manuscript from letters her husband David Kaczynski had received from his older brother Ted, including a 23-page essay in which he raged against the modern world. In the essay, Ted wrote phrases such as, "Technology has already made it impossible for us to live as physically independent beings."
Patrik said she struggled with her suspicions about her husband's brother, but "it was really important to talk to David about it."
"When she said, 'Well, I think maybe your brother's the Unabomber,' I thought, 'Well, this is not anything to worry about. Ted's never been violent. I've never seen him violent,'" David Kaczynski said in an interview for "20/20 on ID Presents Homicide." "I couldn't imagine that he would do what the Unabomber had done."
As Patrik's suspicions continued to grow, she said she went to the library with her husband to read more of the Unabomber's manifesto with him.
"I thought I was going to read the first page of this, turn to Linda and say, 'See, I told you so.' But on an emotional level, it just sounded like my brother's voice. You know, it sounded like the way he argued, the way he talked, the way he expressed an idea," said David Kaczynski.
Ted Kaczynski had attended Harvard at 16 years old and earned a Ph.D. in math at the University of Michigan. But when he was a math professor at U.C. Berkeley, Kaczynski gave up on mainstream society. He built a small cabin in Montana for himself and retreated from the world.
"He began to write very hostile, angry, resentful letters to our parents. I had a hard time understanding where the resentment came from or what justified it in his mind," David Kaczynski recalled.
David Kaczynski said he and his family had long suspected Ted suffered from some kind of mental illness, but they had been in denial for a long time.
"It's pretty clear that by the time he was a graduate student at the University of Michigan, he was suffering from some pretty serious delusions. He stopped going to classes. He would be in his room. He was having delusions that people were laughing about him or making fun of him, or plotting against him," David Kaczynski said.
The family eventually made the wrenching decision to contact the FBI, and on April 3, 1995, a nine-man swat team apprehended Ted Kaczynski in his cabin in Lincoln, Montana. The team found bomb materials, paneling nails, notebooks that contained almost 40,000 pages of writing and the typewriter on which Ted Kaczynski had typed his manifesto.
In the ensuing years, David Kaczynski and Gary Wright, who survived Ted Kaczynski's 12th bomb, have traveled the country speaking out against the death penalty and advocating for reconciliation, mercy and grace instead.
"I think on some level, whether we recognize it or not in ourselves, there is this hunger for reconciliation, [and] that, you know, violence not be the last word," David Kaczynski said.
David Kaczynski said he also remains thankful for his wife voicing her suspicions about his brother.
"I don't know if Linda understands how grateful I am to her for what she did, for her courage," David Kaczynski said. "Linda saved lives. She saved our family's honor and self-respect and, ultimately, perhaps contributed to saving Ted's life, too."