His hair is unchanging through the years, a little boy’s simple bowl cut. His eyebrows do not arch or smile. His lips are neither pursed nor puckered but always flat, giving nothing away. His eyes, year after year, pop out from the snapshots, open wide, connecting with no one.

This is what Adam Lanza’s pursuers are left with. In violent death as in isolated life, he gave away little. The 20-year-old who on a bright Friday morning killed 26 people at the elementary school he had attended, as well as his mother and himself, destroyed one promising key to his unspoken passions, hammering his computer’s hard drive into digital silence. He left no note, confided in no friend. His mother, the one person he was known to have spoken to in anything more than monosyllabic responses, he shot in the head, four times, while she was in bed, in her pajamas.

Like Jared Loughner in Arizona and James Holmes in Colorado, Lanza stares out at us, bug-eyed and disconnected, in the grainy snapshot that is our first window into his soul. Mass shooters, almost always loners, often look the part, meaning that the publicly available images of them portray them in no social context, looking but not seeing, seen but not known.

In the desperate search for motive where madness has prevailed, the Lanza case is more frustrating than most. For a young man who spent most of his waking hours at a computer, he appears to have left behind an astonishingly small online footprint — no Facebook page, no Twitter account.

The Connecticut State Police, assisted by the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, has poured everything it has into the Newtown shootings investigation, but “we don’t have any smoking gun to say this is why it occurred, at least not yet,” said Lt. J. Paul Vance, a department spokesman. “We are looking at several months before we really have our arms wrapped around this.”

Learn more about mass shootings that occurred this year

Investigators have learned through interviews with friends of Lanza’s mother that he spent his days in the windowless basement of the family’s $600,000, 3,100-square-foot house, sitting in front of a screen, anonymously playing violent video games with people he did not know. Lanza’s devotion to the games, rather than to the people playing them, was so single-minded that, across dozens of online gaming sites, his disappearance after thousands of hours of play has left no ripples, no community of people asking what happened to their former competitor.

From the narrow view of the outside world, Adam slipped through the years like sand through an hourglass. He went to a local hair salon occasionally but always in the company of his mother, who answered the hairdresser’s questions for him. He moved from school to school, leaving so little impression that classmates can’t recall which years he was with them. His mother had friends who adored her and saw her frequently, yet they had never been to her house, never met the son she spoke of so warmly.

But inside 36 Yogananda St., Nancy Lanza had grown progressively more worried and frustrated as her younger son withdrew ever more completely from the world, her friends said. She devoted more than a decade to trying to reach him, start him off, cater to his needs, pull him out of himself. By the end, she had realized she wasn’t getting anywhere.

Not known as hostile

Decades later, elementary school teachers tend to remember their students frozen in time, each child a collection of moments, sweet and savory. From Adam Lanza’s time in second grade, Carole MacInnes remembers an intelligence almost lost in silence.

“He was a little boy in my class in second grade” at Sandy Hook Elementary, said MacInnes, who taught in Newtown for 22 years before retiring three years ago. “A thin little fellow. He was very quiet. There was a quiet depth to him that I couldn’t penetrate.”

The distance was unusual for a second-grader, but Adam was not yet debilitated by his withdrawal. “He didn’t need that much from me,” MacInnes said. “Some kids coming in from first grade need more attention, but academically he was fine. Socially, he got along with the others. I don’t remember him as hostile.”

Nancy Lanza visited MacInnes at parent-teacher conferences, and the sessions were unremarkable; the mother had no special concerns.

But Lanza’s worry about her son was already evident, said Wendy Wipprecht, whose son, Miles Aldrich, was invited, along with the rest of their first-grade class, to Adam’s birthday party — duckpin bowling at Danbury Duckpin Lanes. Miles and his mother were happy to be invited, in part because Miles, who had autism and a teaching aide devoted to him, was not always included in class social events.

At the party, Nancy Lanza approached Wipprecht, evidently worried about her own son.

“I got into a long talk with his mother,” Wipprecht said. “She was concerned about Adam. He was obviously very bright and very shy. She was worried he wasn’t doing as well as he should be.”

Wipprecht was surprised by the level of Nancy’s concern. “I didn’t see autism there,” she said.

Several years later, when the boys entered middle school, Lanza took her son out of the public system and put him into St. Rose of Lima, a Catholic school, because “she thought he would do better in smaller classes,” Wipprecht said.

The two women did not stay in touch after the boys finished elementary school. Their boys did not see each other again until 10th grade at Newtown High School, where Miles remembers Adam being “very, very quiet,” Wipprecht said.

Somewhere along the way, Adam was given a diagnosis of Asperger syndrome, a developmental disorder related to autism that usually involves impaired social skills, difficulty communicating, and repetitive and fixated behavior. H. Wayne Carver II, Connecticut’s chief medical examiner, said his office is consulting with geneticists in a search for “any identifiable disease associated with” Adam, noting that “Asperger’s is not associated with behavior patterns that are violent.”

At St. Rose — although the Lanzas were Catholic, they were not religious and did not belong to the church, friends and relatives said — Adam “wasn’t interested in what normal, average 13-year-olds were interested in,” said Nicholas Martinez, a classmate who is now a student at Seattle University. “He didn’t like contemporary music” but listened to classic rock from the 1950s.

Students who knew Adam at St. Rose school said he was distant but not entirely unreachable. “He wasn’t an outcast, but he didn’t fit in either,” Martinez said.

“You would say ‘hi,’ and he would say ‘hi’ back, but he didn’t give you a lot to work with,” said Kate Leen, now a student at Hofstra University.

In ninth grade, Adam returned to public school, but his difficulty relating to people was obvious even to those who had only casual contact with him. Newtown High, one of the largest schools in Connecticut with 1,750 students, reflects the town’s affluence — the median family income is $104,000 — with 87 percent of students planning to attend college and test scores well above the state average.

A high school classmate who was on the honor roll with Adam in 10th grade saw his photo after the shootings and didn’t recognize the young man. Then she saw an older picture, from Newtown High’s tech club, which is where she came to know Adam.

“I knew exactly who it was,” she said, talking on the condition of anonymity.

Lanza was the only kid at Newtown High who dressed as he did — Dockers, polo shirts, something like “business casual,” said the classmate. Adam’s shirt was often untucked. His clothes never quite fit, hanging off him, “loose. They were nice clothes but ill-fitting. It made for a funny picture. It would look strange.”

Adam carried a briefcase every day. No other students did.

Newtown High was a cold place for a kid like Adam, according to his peers and their parents. He was alone and apart from the school’s intricate social circles and the swirl of teenager politics.

The classmate said she, too, was nerdy and had a tough time with the popular kids. “No one went out of their way to include me,” she said. “Anyone who was different didn’t fit in. If you let it get to you, it would destroy you.”

She would see Adam in the cafeteria, sitting near her small group of friends, invariably surrounded by people but never saying a word to any of them.

“This is a person you would have had to watch and care for and protect him from the other students,” said Richard Novia, who was security director for the Newtown schools for 16 years. “This is a boy who always had a buttoned shirt right to the top button.”

Adam was not only “a quiet, shy child, small in stature . . . and challenged to make friends,” but also regularly had episodes in which “he’d just shut down and pull within himself,” Novia told the Associated Press. “He would avoid verbal or physical contact with just about anyone if he could. Getting him back out of that would be challenging.”

On such occasions, the school would contact his mother, whom Novia saw as “an excellent parent,” and she could coax her son out of his shell.

Novia also knew Adam through the high school’s tech club, which brought together teens who played computer games and wrote programming code.

Club members said they would gather at one another’s houses to link their laptops and play games at LAN parties, named for the local area network that connected their computers. According to club members, Adam played “Star Craft” and “War Craft III: Reign of Chaos,” in which, as the manufacturer puts it, a dark “shadow has fallen over the world, threatening to extinguish all life — all hope.”

Adam once hosted a LAN party, according to Gloria Milas, whose son Joshua sometimes played computer games with Adam.

In class, Adam did his work and said little or nothing. He left Newtown High during 10th grade, never to return, peers said. Adam finished high school through self-study at home, graduating in 2009. That year, at age 16, he began taking courses at Western Connecticut State University in Danbury.

Adam took six classes at the college in 2008 and 2009, in history, computer science and German. His grades were good — A’s and B’s for a 3.26 average — but he made little impression on his professors and left campus without having grown close to anyone there.

Cutting some family ties

Adam left school around the time his parents finalized their divorce. His father, Peter Lanza, a vice president of taxes at GE Energy Financial Services, had moved out of the Newtown house in 2001; in 2009, the parents completed the state-required mediation and parenting classes.

An acquaintance who spent time in the Lanzas’ house around the time of the divorce described the breakup as friendly and respectful, without visible rancor.

“They always stayed civil,” said Marsha Lanza, who is married to Peter Lanza’s brother Michael. “They always stayed friends.”

Nancy, who had worked as a stockbroker in Boston before she had children, got the house and alimony that started at $240,000 a year and rose over six years to about $300,000 — well more than half of Peter’s gross salary. The couple agreed that Adam would live with his mother, who would have final say on decisions regarding his education and care, just as she had during the Lanzas’ years of separation.

The father, who saw Adam regularly on weekends during the years of separation, remarried in 2010, to a librarian at the University of Connecticut. Friends and relatives said Adam cut off contact with his father and his brother, Ryan, around that time.

Adam hadn’t spoken to his father or older brother in at least two years, according to the source who has been in touch with the Lanza family last week. Peter and Ryan Lanza have not publicly responded to reporters’ questions.

Alone with each other in a big house with a pool on two acres, mother and son struggled over issues of independence, friends said. In 2010, Adam got a driver’s license but did not go out by himself often.

Nancy remained dedicated to Adam, catering to his heightened sensitivities.

“I believe she thought she had it under control,” Marsha Lanza said. “I believe if the kid needed help, she would have gotten it, because that’s who she was. They had the money. She had time because she was a stay-at-home mom. She wasn’t afraid to be there for her kids. She was involved. That’s why when I heard that he shot her, that floored me. Your mom did all this stuff for you; what the hell were you thinking? Why did you take your revenge out on her? What did she do? So there’s something I don’t know or I’m missing.”

Adam would sometimes “isolate himself,” Nancy’s friend Ellen Adriani told NBC News. “One time he was ill, and he just didn’t want her in the room. So she stayed outside all night on the carpet of his bedroom. He periodically would say: ‘Are you there? Are you there?’ And she’d always say, ‘Yes, I’m here.’ So he wanted her there to some degree, but not in his exact, immediate space.”

Adam — 5-foot-10 and thin, with blue eyes, according to his driver’s license — had become a vegan and insisted on eating organic food. Family friends said he was politically conservative, although he was the one member of his immediate family not registered to vote. He developed impressive speed and moves on the arcade game “Dance Dance Revolution,” which he would play at a local game store, sometimes drawing a clot of onlookers. But if a stranger tried to join him in what is usually a two-person game, Adam would walk away from the machine and out of the store.

In the past couple of years, Nancy searched for ways to get Adam out of the house, away from the vast collection of violent video games he kept in one of his two bedrooms. One of those rooms was in the basement, not far from the lockbox where Nancy kept her firearms — at least five of them, all purchased legally, all obtained after her divorce.

An acquaintance who spent time in the Lanza house in earlier years said he had seen no sign of guns or interest in weaponry at that point.

The weapons were for “self-defense,” said Marsha Lanza. “She lived alone. She was a female, lived alone.”

Nancy, who grew up on a farm in New Hampshire in a family that owned guns, believed that teaching Adam how to shoot would give him “a sense of responsibility,” her friend Russell Hanoman said on NBC News. “Guns require a lot of respect, and she really tried to instill that responsibility within him, and he took to it. He loved being careful with them.”

Mother and son went shooting together at local ranges on multiple occasions, law enforcement officials said.

Adriani told the Connecticut Post that in the past couple of years, Adam had been eager to join the Marines or some other branch of the military, but Nancy Lanza, despite initial enthusiasm for the idea, concluded that her son could not thrive in the service. After all, he could not stand to be touched and had trouble speaking to others.

More recently, Nancy had told friends this fall that she was considering moving Adam to Washington state to enroll him in a school that she believed could help him.

Adam was 20 now, and Nancy told friends she was eager to find ways for him to get out of the house. She had taken some trips without him, leaving his meals prepared for him. She was on one of those trips, by herself, for the three days before the shootings, staying at the Mount Washington Resort and Hotel in Bretton Woods, N.H., according to local news reports.

Law enforcement sources said Adam was dressed in black clothes covered with body armor when he burst into Sandy Hook Elementary School on Dec. 14. He had his mother’s Bushmaster rifle and her Glock and Sig Sauer handguns.

In a matter of minutes, he killed 20 children in the school he once attended. He killed six adults who cared for those children in the same rooms where Carole MacInnes and other teachers sought to connect with him. He had already killed the only person who could draw him out of his hard shell. And then he took his mother’s handgun, pointed it at his face and blew himself out of this world.

Fisher and O’Harrow reported from Washington, Finn from Newtown. Tim Craig, Peter Hermann and Michael Rosenwald in Newtown; Brady Dennis, Sari Horwitz, Julie Tate, Alice R. Crites and Jennifer Jenkins in Washington; and Kari Lydersen and Peter Slevin in Crystal Lake, Ill., contributed to this report.