He was a "shut-in," a young man with a twisted murder obsession holed up in a suburban house with guns, Samurai swords and a mother who searched self-help books for solutions to his social disorder.
That's the picture that emerged Thursday of Adam Lanza as search warrants carried out after his Dec. 14 massacre at Sandy Hook Elementary School were made public.
The documents provide no clear motive for the two-part rampage that left 20 children, six school staffers, Lanza's mother Nancy and the gunman dead, but they hint at the activities that consumed his days behind dark-green shutters on Yogananda St. in Newtown, Conn.
One piece of paper seized from the home is particularly chilling in hindsight: a 2008 New York Times clipping about a shooting at Northern Illinois University, where a gunman murdered five people, wounded another 21 and then killed himself.
Although it's not spelled out in the warrants, a law-enforcement source told NBC News that police also found a spreadsheet that Lanza toiled over, cataloging the details of mass murders through the years.
Police also discovered Lanza's journals, though the warrants don't divulge if they contained any clues about why the 20-year-old slaughtered defenseless first-graders or how long he had planned the shooting spree.
There was a large assortment of computer equipment, including a custom-built desktop unit — not surprising since Lanza reportedly earned an A in computers as a 16-year-old freshman at Western Connecticut State University and worked for a time at a computer store.
More notable were missing and damaged hard drives, which law-enforcement experts saw as a sign that Lanza didn't want police to examine his computing history after he joined the nation's growing roster of mass killers.
The electronics seized included a Xbox system, and the warrants quoted an anonymous tipster who told the FBI that Lanza was "an avid gamer who plays Call of Duty."
Most startling was the array of weapons found at the Lanza home and at the school: a half-dozen handguns and rifles, a BB gun, a starter pistol, hundreds of rounds of ammunition scattered about, high-capacity magazines, three Samurai swords, a bayonet and smaller knives in sheaths.
"It's a stunning amount of ammunition and weaponry," said Mary Ellen O'Toole, a former FBI profiler.
"If the family dynamic is gun-oriented, that's fine. But how do they treat it? Are their weapons locked up? Is the ammunition kept in the same place? These documents tell us this is not the case. You've got this stuff laying all around and it's not stored properly."
The warrants also reveal that bullets were kept in a Planters nut canister and plastic baggies, in a bedroom gun safe, on closet shelves, in a shoe box, a duffel bag, and a filing cabinet drawer.
Family friends and acquaintances have said that Nancy Lanza, who grew up in rural New Hampshire, saw recreational shooting as something she could do to bond with Adam and his older brother, Ryan.
A holiday card found in the house underscores that connection: it contained a check that Nancy wrote to Adam for the purchase of a firearm, according to the warrants.
Mother and son both had documents described as National Rifle Association certificates, though it was unclear what that signified, along with files of gun-related receipts, manuals and other paperwork.
Had the guns and ammunition been kept in a safe place and had Adam Lanza been a well-adjusted person with friends and outside interests, the arsenal might not have raised any eyebrows, O'Toole said.
But Lanza didn't fit that description.
The FBI tipster told agents the suspect "rarely leaves his home and considers him to be a shut-in," according to the warrants.
An extensive profile of the shooter in the Hartford Courant last month chronicled how Lanza cut himself off from others in the last two years of his life — following his parents' divorce and an abrupt end to his education, which had been a patchwork quilt of mainstream and special-education classes and home-schooling.
The FBI's source said school had been Lanza's "life" and that he once attended Sandy Hook. After moving from New Hampshire to Newtown, Lanza entered the first grade there. A report card from Sandy Hook was found in the home.
Over the next decade, Lanza was shuttled in and out of classrooms by his mother, who believed he had sensory integration disorder and needed independent study at home, the Courant reported.
Lanza hated to be touched, had few friends and was easily freaked out by changes in routine, the newspaper said. In middle school, he was diagnosed with Asperger's syndrome, a condition on the autism spectrum marked by social awkwardness and obsessive interests.
Proof that Nancy Lanza was still looking for insight into her son's behavior years after that diagnosis could be found on her home's bookshelves. Among the titles seized by police were a primer on Asperger's and another on autistic savants.
A third book, "Train Your Brain to Get Happy," had pages tabbed off, though it was unknown if mother or son had been looking for the "joy, optimism and serenity" promised in the subtitle.
Those feelings appeared to be elusive at the two-story yellow Colonial, where investigators found three gruesome pictures of a blood-spattered body under plastic. The origin of the photos was not specified.
O'Toole said the warrants reveal there was no shortage of warning signs for Nancy Lanza that her younger son was headed down a dark path.
"But it takes a big step for a lot of people who love their children to go from, 'I think i have a problem' to 'I think this person could commit homicide,'" she said. "It's not unusual for people to ignore behavior, explain it away or to normalize it or to rationalize it."
Not unusual — but in this case, fateful.
Among the other items ticked off in the warrants were two bloody sheets, apparently from the bed where Nancy Lanza was killed with a .22-caliber round to the forehead while sleeping, just before her son loaded her Honda Civil with handguns, rifles and bullets and, police say, went hunting for innocent children.
NBC News' Michael Isikoff contributed to this report