Connecticut gunman Adam Lanza has been described as "socially awkward", "shy", "a nerd" and "super smart".
As a teenager he would scuttle from class to class, pressing himself against walls and clutching a black briefcase "like an eight-year-old with a teddy bear".
What has emerged in the aftermath of the Sandy Hook shooting is a picture of an odd, withdrawn and troubled boy.
One law enforcement officer has said that 20-year-old Lanza had been diagnosed with Asperger's, a mild form of autism characterised by social awkwardness.
People with the disorder are often highly intelligent and can be frustrated more easily but there is no link to violent behaviour.
According to Richard Novia, adviser at the Newton High School tech club, Lanza's mother Nancy would come to the school to deal with episodes her son would suffer.
He said Lanza experienced "total withdrawal from whatever he was supposed to be doing, be it a class, be it sitting and reading a book".
He would press himself against a wall or walk in a different direction when others approached and that he never seemed to feel pain, Mr Novia said.
However he added: "Somewhere along in the last four years there were significant changes that led to what has happened Friday morning. I could never have foreseen him doing that."
Former classmate Alex Israel said: "He was always different - keeping to himself, fidgeting and very quiet. But I could always tell he was a super smart kid, maybe just socially awkward, something just off about him.
"The same went for when I went to his house. His mother was always nice to me; she was a kind, typical suburban mom as far as I remember."
She told the New York Times: "I'd see him in the halls walking quickly with his briefcase he carried, but I never had a class with him and never saw him with friends."
Tim Arnone, who was in the tech club with Lanza, said: "It was definitely the nerdiest club in the school. We called it the tech club. We had our own little section in the room."
He said Lanza had been "driven hard" to succeed academically by his parents and added: "She (his mother) pushed him really hard to be smarter and work harder in school."
The gunman's aunt Marsha said her nephew, who lived in a large colonial-style home with his mother in Newtown, was raised by kind, nurturing parents who would not have hesitated to seek mental help for him if he needed it.
She described Nancy Lanza, a former stockbroker, as a good mother and kind-hearted. If her son had needed counselling, "Nancy wasn't one to deny reality," she said.
It was from Ms Lanza, though, that her son may have picked up his interest in guns. The 52-year-old was a gun enthusiast who used to take her sons to the shooting range with her.
She owned five weapons: two handguns, two traditional rifles and a semi-automatic, similar to ones used by soldiers in Afghanistan.
Lanza, who had no criminal record, took the handguns and the semi-automatic when he set out to Sandy Hook School, where he was once a pupil, on Friday.
Dan Holmes, the owner of a landscaping firm, described Ms Lanza as an avid gun collector and said she had once shown him a "high-end rifle" she had bought.
"She said she would often go target shooting with her kids," he said.
As investigators continue to search for a motive, Lanza's family is struggling to understand what made him shoot his mother dead, take her guns, then smash his way into the local elementary school and gun down 26 people.
Lanza's brother, Ryan, has been questioned by police but said he had not been in touch with his brother since about 2010.
James Champion, Nancy Lanza's brother and a retired police captain, said he had not seen his nephew for eight years.
He said in a statement: "On behalf of Nancy's mother and siblings, we reach out to the community of Newtown to express our heartfelt sorrow for the incomprehensible loss of innocence that has affected so many."
Lanza's father expressed his sympathies for the families of the victims but was at a loss to explain his son's actions.
In a statement he simply said: "We are in a state of disbelief and trying to find whatever answer we can. We too are asking why."