By the time Ted Kaczynski was arrested on April 3, 1996, he had perpetrated a 17-year bombing campaign that left three people dead and 23 injured. He had delivered or mailed bombs to universities and computer stores, to lobbyists and advertising executives. The death toll as a result of his actions would have been much higher, but, in 1979, the bomb he placed in the hold of American Airlines Flight 444 failed to explode. Soon afterward, the FBI became involved, and assigned the case a catchy code name — UNABOM (university and airline bomber) — that didn’t hurt the bomber’s growing mystique. Yet Kaczynski, working alone in a cabin without plumbing or electricity in the woods outside Lincoln, Montana, continued to evade identification. By 1995, the UNABOM investigation had become the longest and most expensive manhunt in American history, and had produced no suspects.

Kaczynski’s mistake — but also, in his own eyes, the culmination of his campaign — came that year when, after nearly two decades of almost complete silence, he suddenly became talkative. He began writing letters to his victims and targets, and to newspapers and magazines, explaining some of his motives (“the industrial-technological system has got to be eliminated”), dispensing warnings (“it would be beneficial to your health to stop your research in genetics”), and mocking the investigation into his crimes (“the FBI is a joke”). In September 1995, on the basis of the bomber’s commitment to “desist from all terrorist activities,” both The New York Times and The Washington Post printed his 35,000-word anti-technology manifesto, Industrial Society and Its Future.

By this point, the Unabomber was a star. Websites and forums proliferated to celebrate what one participant called “the mystical and awe-inspiring Unabomber.” A political action committee was formed to promote a “Write-In For President Unabomber ’96.” He was likened to 19th-century outlaws like Jesse James — but others said he was more than that: he was a philosophical criminal, and one who said things that were in tune with the anxieties of modernity. “There’s a little bit of the Unabomber in most of us,” Robert Wright wrote in Time. “We may not share his approach to airing a grievance, but the grievance itself feels familiar.”

Yet for all this attention, David Kaczynski had hardly noticed the case. “I don’t remember even hearing the word ‘Unabomber’ prior to December 1994,” he admits in his brief and affecting memoir, Every Last Tie: The Story of the Unabomber and His Family. He had never considered his brother capable of violence, let alone serial murder. Ted, he believed, was strange, hermetic, maybe even unstable, but “my only fear along those lines was the haunting worry that he might someday kill himself.” It was at the urging of his wife, Linda Patrik, that he considered the possibility that Ted might be involved. He read the manifesto, and admitted that he saw some similarities: Ted held strong views about technology and society, and he had been known to use unusual phrases such as “cool-headed logicians” that appeared in the manifesto. But there was no “smoking gun,” and David was terrified that if he turned Ted in he would be executed, or that the FBI’s attempt to arrest him would end in disaster (the Waco siege was fresh in his mind).

He started looking for proof that his fears about Ted were unfounded. A well-known police sketch — a pulp depiction of a young man in aviator sunglasses and a hoodie who looked like a “super villain” according to a Unabomber t-shirt salesman — didn’t resemble Ted, and the description that accompanied it was of a man with different color hair, who was 10 years younger and three inches taller. Imaginative forensic profiles pegged the bomber as a man without an advanced degree, who probably worked for an airline or in a blue-collar job. Ted Kaczynski was educated at Harvard, earned a PhD from the University of Michigan, and once held a tenure-track position at University of California, Berkeley. Nor was the manifesto unequivocally Ted’s: he had never complained much about “leftists,” for instance, whereas the bomber seemed to have a deranged hatred for them.

But David couldn’t convince himself. He contacted a private investigator — something not mentioned in Every Last Tie — who found strong evidence that Ted’s writing style matched the bomber’s, and that he was in the relevant cities at the right times. In November 1995, desperately hoping to find some conclusive proof that his brother was not involved, he wrote to Ted and asked to visit him in Montana. He quotes Ted’s response in Every Last Tie:

I get just choked with frustration at my inability to get our stinking family off my back once and for all, and “stinking family” emphatically includes you. […] I DON’T EVER WANT TO SEE YOU OR HEAR FROM YOU, OR ANY OTHER MEMBER OF OUR FAMILY AGAIN.

Ted had nursed an intense hostility toward his parents for years, but it was this letter, David writes, that convinced him that his brother had “gone over the edge”:

Ted’s anger and coldness were truly incomprehensible to me. I couldn’t fathom what was going on inside his mind […] I realized I couldn’t reason with him; I couldn’t control him. And now it struck me that he probably could not control himself either.

He took his suspicions to the FBI; after all the resources dedicated to the case, Ted Kaczynski was caught because his own brother turned him in.

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The question of what it means to have “gone over the edge” came to define the trial. Since Ted Kaczynski’s guilt was easy to establish — in his cabin, investigators found a bomb and a copy of the manifesto, as well as journals carefully documenting his campaign from beginning to end — and because, over David’s horrified objections, the prosecutors elected to seek the death penalty, Ted’s legal representatives and his family believed they had no option but to portray Ted as mentally ill. They retained psychiatrists such as Karen B. Froming, who administered neuropsychological tests that “revealed deficits of a mild nature” in various areas of cognitive and affective processing. His lawyers also cited Ted’s reclusive lifestyle — living alone in a primitive cabin without any long-term relationships — as evidence of his unbalanced mental state.

David Kaczynski threw himself into this campaign, and emerged as a sophisticated advocate. He gave interviews to The New York Times, The Washington Post, and 60 Minutes that sought to portray Ted as a good person who had gone astray because of mental deficiencies or psychological traumas in his childhood. “He had this incredible capacity to show human sympathy,” he told the Times, describing Ted’s heartfelt letters to bereaved family members and his concern for David’s well-being after a sawing accident. Every Last Tie picks up where that interview, published in 1996, leaves off: Kaczynski relates events that he thinks might have contributed to his brother’s instability, such as a hospital stay that separated him from his parents as a baby, and traumatic psychological trials he was subjected to at Harvard (these were part of the CIA’s controversial Project MKUltra on interrogation techniques; the trials Ted took part in, which he would later describe as “the worst experience of my life,” involved subjecting participants to “vehement, sweeping and personally abusive attacks”). This search for explanations is understandable, of course — but ultimately futile. Even David, who makes clear he is inclined to believe the psychohistorical explanations, eventually concludes that for all his searching, “perhaps a genetic flaw, a predisposition to mental illness, would have taken Ted down eventually in any case.”

What aggravated some observers of the trial was what they perceived as an attempt to blur explanation and exculpation: to reduce a fanatical criminal to the victim of an illness for which he bore no responsibility. Many who took this view wanted to have him executed, and the paradox of the trial was that Ted took their side. The letter that convinced David that his brother had gone over the edge also contained these lines (not quoted in Every Last Tie): “I am not ‘suffering, sick or discouraged,’ and I don’t know what ‘indications’ you think you have that I am so.” His dismay at the idea of being labeled mentally ill derived from a fear it would discredit his views and motives. His very intransigence on the subject was taken as evidence of his paranoid mental state. What emerged was a strange alliance of opponents of Kaczynski who believed that finding him insane let him off the hook, and supporters of Kaczynski (or at least sympathizers with his manifesto) who believed that finding him insane did a disservice to the seriousness of his ideology.

This ideology was by far the most controversial issue in relation to Ted’s sanity. David suggests that his family had long feared Ted’s views were a form of rationalization: a scaffolding of delusions to protect himself from a world he couldn’t relate to or understand. It wasn’t just the family. Froming told William Finnegan of The New Yorker that, as much as any test she’d administered, it was Ted’s ideas, his “view of technology as the vehicle by which people are destroying themselves and the world,” that indicated a diagnosis of paranoid schizophrenia. Meanwhile, the prosecutors planned to use precisely these views as evidence of his long-term, calculated plan to carry out murders and destruction. Park Dietz, a forensic psychiatrist, read Ted’s journals and told Finnegan:

They’re full of strong emotions, considerable anger, and an elaborate, closely reasoned system of belief about the adverse impact of technology on society. The question always is: Is that belief system philosophy or is it delusion? The answer has more to do with the ideology of the psychiatrist than with anything else.

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Every Last Tie mostly avoids the issue of Ted’s ideology. “He posed questions,” David writes, “important questions about humanity’s future — that no one who has truly understood them can answer.” He also mentions his own stint during the 1980s alone in the Texas desert, where he retreated after becoming convinced that “the world’s technological culture [was] fatally contaminated by nihilism.” But that’s about it: enough to tell us that he’s fundamentally in agreement with his brother about the problems at hand, and not much more. This might be a matter of message control: David has never strayed from the belief that his brother is mentally ill, but he can’t quite hold the line on whether Ted’s views about technology were a serious critique or mere rationalization, so he avoids the issue.

Nevertheless, it would be wrong to reduce the reading of a book like this to an exercise in untangling the rhetoric from the reality. The book isn’t about technology or the ideas Ted tried to spread through bombing; it’s about what it’s like to realize your brother has committed serial murder. David Kaczynski writes with a reserved power, and some of the most affecting scenes — his father’s suicide, for example — have nothing to do with Ted. He mocks the idea of himself as a moral hero for turning his brother in (a lesser person would have reveled in that) and he’s aware of the irony of his position: “Ted’s cruelty stigmatizes my good name; but my reputation for goodness comes at his expense.” Of course, it’s also true that David’s good reputation is a result of the success of his campaign to present Ted as mentally ill — an aberration, not a representation of the Kaczynski family.

For his part, Ted Kaczynski has always stood by his convictions. He was so serious about avoiding an insanity defense that he tried to replace his legal team with J. Tony Serra, a civil rights lawyer known for defending controversial clients, who was willing to present an ideological defense — despite the fact that it would drastically increase his chances of receiving the death penalty. When the judge refused to grant the request, on the grounds that Serra would need months to prepare, Kaczynski decided to invoke his Sixth Amendment rights to represent himself. Again, the judge refused on the same grounds. It was at this point, believing that he had been backed into a corner, that Kaczynski elected to strike a plea bargain: in exchange for a guilty plea, with no possibility of appeal, he would be sentenced to life without parole, but avoid the death penalty. He would also avoid a humiliating court battle centered on a dispute about his mental health.

Since then, Ted Kaczynski has occupied himself with writing and with voluminous correspondence. Looking through some of it, you get an impression of the force of his fixation. He exchanges a pleasant series of letters with a man writing a trail guide, then adds that the man might want to use his guide as an opportunity to say something about “why the invention of civilization was the greatest mistake the human race ever made.” He would have supported 9/11 had he thought it was about opposing modernity rather than gaining power. He would also have supported President George W. Bush, not just because of his opposition to stem cell research, but also because he figured “the re-election of an incompetent president and his irresponsible gang will help weaken the system.”

He’s as determined as ever to present this as ideology, not delusion — and it’s fair to say that he won’t have appreciated David’s renewed attention. Several journalists have received letters from Kaczynski since the publication of Every Last Tie, including New Yorker staff writer Lawrence Wright and Bryan Denson, who was one of dozens of journalists seeking an interview with Ted after his arrest (“How many ways can esteemed journalists suck up to a serial killer?” Howard Kurtz asked in The Washington Post). The letter offers an interview about his brother’s “recent comments” and how “they are being used to torment me.” Only one person will receive this interview: the one who can most convincingly claim that he or she doesn’t believe he is insane.

Kaczynski later claimed the letters were a hoax perpetrated by “some creep in Santa Barbara.” But it’s no great loss. It’s not clear what more there is to learn about the Unabomber. After all, if nothing else, Ted Kaczynski is consistent.

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Ben Jackson has written for the London Review of Books, The Awl, and The Guardian.