In Peter Lanza’s new house, on a secluded private road in Fairfield County, Connecticut, is an attic room overflowing with shipping crates of what he calls “the stuff.” Since the day in December, 2012, when his son Adam killed his own mother, himself, and twenty-six people at Sandy Hook Elementary School, strangers from across the world have sent thousands upon thousands of letters and other keepsakes: prayer shawls, Bibles, Teddy bears, homemade toys; stories with titles such as “My First Christmas in Heaven”; crosses, including one made by prison inmates. People sent candy, too, and when I visited Peter, last fall, he showed me a bag of year-old caramels. He had not wanted to throw away anything that people sent. But he said, “I was wary about eating anything,” and he didn’t let Shelley Lanza—his second wife—eat any of the candy, either. There was no way to be sure it wasn’t poisoned. Downstairs, in Peter’s home office, I spotted a box of family photographs. He used to display them, he told me, but now he couldn’t look at Adam, and it seemed strange to put up photos of his older son, Ryan, without Adam’s. “I’m not dealing with it,” he said. Later, he added, “You can’t mourn for the little boy he once was. You can’t fool yourself.”

Since the shootings, Peter has avoided the press, but in September, as the first anniversary of his son’s rampage approached, he contacted me to say that he was ready to tell his story. We met six times, for interviews lasting as long as seven hours. Shelley, a librarian at the University of Connecticut, usually joined us and made soup or chili or salads for lunch. Sometimes we played with their German shepherd. When Peter speaks, you can still hear a strong trace of rural Massachusetts and southern New Hampshire, where he and his first wife—Nancy, Adam’s mother—grew up. He is an affable man with a poise that often hides his despair. An accountant who is a vice-president for taxes at a General Electric subsidiary, he maintains a nearly fanatical insistence on facts, and nothing annoyed him more in our conversations than speculation—by me, the media, or anyone else. He is not by nature given to self-examination, and often it was Shelley who underlined the emotional ramifications of what he said.

Peter hadn’t seen his son for two years at the time of the Sandy Hook killings, and, even with hindsight, he doesn’t think that the catastrophe could have been predicted. But he constantly thinks about what he could have done differently and wishes he had pushed harder to see Adam. “Any variation on what I did and how my relationship was had to be good, because no outcome could be worse,” he said. Another time, he said, “You can’t get any more evil,” and added, “How much do I beat up on myself about the fact that he’s my son? A lot.”

Depending on whom you ask, there were twenty-six, twenty-seven, or twenty-eight victims in Newtown. It’s twenty-six if you count only those who were murdered at Sandy Hook Elementary School; twenty-seven if you include Nancy Lanza; twenty-eight if you judge Adam’s suicide a loss. There are twenty-six stars on the local firehouse roof. On the anniversary of the shootings, President Obama referred to “six dedicated school workers and twenty beautiful children” who had been killed, and the governor of Connecticut asked churches to ring their bells twenty-six times. Some churches in Newtown had previously commemorated the victims by ringing twenty-eight times, but a popular narrative had taken hold in which Nancy—a gun enthusiast who had taught Adam to shoot—was an accessory to the crime, rather than its victim. Emily Miller, an editor at the Washington Times, wrote, “We can’t blame lax gun-control laws, access to mental health treatment, prescription drugs or video games for Lanza’s terrible killing spree. We can point to a mother who should have been more aware of how sick her son had become and forced treatment.”

Inadequate gun control and poor mental-health care are problems that invariably define the debate after atrocities such as the one at Newtown. But, important as those issues are, our impulse to grasp for reasons comes, arguably, from a more basic need—to make sense of what seems senseless. When the Connecticut state’s attorney issued a report, in December, CNN announced, “Sandy Hook killer Adam Lanza took motive to his grave.” A Times headline ran “CHILLING LOOK AT NEWTOWN KILLER, BUT NO ‘WHY.’ ” Yet no “motive” can mitigate the horror of a bloodbath involving children. Had we found out—which we did not—that Adam had schizophrenia, or had been a pedophile or a victim of childhood abuse, we still wouldn’t know why he acted as he did.

Interview subjects usually have a story they want to tell, but Peter Lanza came to these conversations as much to ask questions as to answer them. It’s strange to live in a state of sustained incomprehension about what has become the most important fact about you. “I want people to be afraid of the fact that this could happen to them,” he said. It took six months after the shootings for a sense of reality to settle on Peter. “But it’s real,” he said. “It doesn’t have to be understood to be real.”

Adam Lanza was never typical. Born in 1992, he didn’t speak until he was three, and he always understood many more words than he could muster. He showed such hypersensitivity to physical touch that tags had to be removed from his clothing. In preschool and at Sandy Hook, where he was a pupil till the beginning of sixth grade, he sometimes smelled things that weren’t there and washed his hands excessively. A doctor diagnosed sensory-integration disorder, and Adam underwent speech therapy and occupational therapy in kindergarten and first grade. Teachers were told to watch for seizures.

Still, photos show him looking cheerful. “Adam loved Sandy Hook school,” Peter said. “He stated, as he was growing older, how much he had liked being a little kid.” Adam’s brother, Ryan, four years older and now a tax accountant in New York, used to joke about how close Peter and Adam were. They’d spend hours playing at two Lego tables in the basement, making up stories for the little towns they built. Adam even invented his own board games. “Always thinking differently,” Peter said. “Just a normal little weird kid.”

Even in an age when a child’s every irregularity is attributed to a syndrome, the idea of a “normal weird kid” seems reasonable enough, but there were early signs that Adam had significant problems. He struggled with basic emotions, and received coaching from Nancy, who became a stay-at-home mother after Adam was born. When he had to show feelings in a school play, Nancy wrote to a friend, “Adam has taken it very seriously, even practicing facial expressions in the mirror!” According to the state’s attorney’s report, when Adam was in fifth grade he said that he “did not think highly of himself and believed that everyone else in the world deserved more than he did.” That year, Adam and another boy wrote a story called “The Big Book of Granny,” in which an old woman with a gun in her cane kills wantonly. In the third chapter, Granny and her son want to taxidermy a boy for their mantelpiece. In another chapter, a character called Dora the Berserker says, “I like hurting people. . . . Especially children.” Adam tried to sell copies of the book at school and got in trouble. A couple of years later, according to the state’s attorney’s report, a teacher noted “disturbing” violence in his writing and described him as “intelligent but not normal, with anti-social issues.”

Meanwhile, Peter and Nancy’s marriage was starting to unravel. Peter’s own father had been relatively disengaged from his wife and buried himself in work, and Peter didn’t have a strong model for family life. “I’d work ridiculous hours during the week and Nancy would take care of the kids,” he told me. “Then, on the weekends, she’d do errands and I’d spend time with the kids.” Peter frequently took the boys on weekend hiking trips. In 2001, Peter and Nancy separated. Adam was nine; when a psychiatrist later asked him about it, he said that his parents were as irritating to each other as they were to him.

Peter recalled, “The funny part is that the separation didn’t really change things for the kids very much.” He moved to Stamford, nearly an hour from Newtown, but still saw the boys every weekend. When Adam entered middle school, he proudly took Peter to see it. “And talk about talkative: man, that kid, you couldn’t shut him up!” Peter said. In the years that followed, they would talk about politics. Adam was a fan of Ron Paul, and liked to argue economic theory. He became fascinated with guns and with the Second World War, and showed an interest in joining the military. But he never talked about mass murder, and he wasn’t violent at school. He seldom revealed his emotions, but had a sharp sense of humor. When Peter took him to see Bill Cosby live, Adam laughed for an hour straight. He loved reruns of “The Bob Newhart Show” and “Get Smart,” which he would watch with his dad. One Christmas, Adam told his parents that he wanted to use his savings to buy toys for needy children, and Peter took him shopping for them.

When Adam began middle school, Peter and Nancy’s worries increased. The structure of the school day changed; instead of sitting in one classroom, he had to move from room to room, and he found the disruption punishing. Sensory overload affected his ability to concentrate; his mother xeroxed his textbooks in black-and-white, because he found color graphics unbearable. He quit playing the saxophone, stopped climbing trees, avoided eye contact, and developed a stiff, lumbering gait. He said that he hated birthdays and holidays, which he had previously loved; special occasions unsettled his increasingly sclerotic orderliness. He had “episodes,” panic attacks that necessitated his mother’s coming to school; the state’s attorney’s report says that on such occasions Adam “was more likely to be victimized than to act in violence against another.”

“It was crystal clear something was wrong,” Peter said. “The social awkwardness, the uncomfortable anxiety, unable to sleep, stress, unable to concentrate, having a hard time learning, the awkward walk, reduced eye contact. You could see the changes occurring.” It is hard to be sure whether new problems were setting in or old ones were becoming more apparent. Michael Stone, a psychiatrist who studies mass murder, said that, as children grow up and tasks become more difficult, what seems like a minor impairment becomes major. “They’re a little weird in school. They don’t have friends. They do not get picked for the baseball team,” he said. “But, as they get to the age when kids begin to date and find partners, they can’t. So the sense of deficit, which was minor in grade school, and getting to be a little bit more in junior high, now becomes very acute.” He added that, without the brain getting worse, “life challenges nudge them in the direction of being sicker.”