Adam Lanza, the 20-year-old who killed his mother and 26 people at a primary school in Connecticut, was "very, very bright", his aunt claimed as she disclosed his mother had chosen to home school him after "battling" education authorities.
Marsha Lanza, the gunman's aunt, said her own children "know right from wrong" and insisted "you've got to give your kids roots" as she spoke out following the shootings at Sandy Hook primary school on Friday.
In her first interview, she disclosed details of his mother Nancy's "battles" with education authorities over Adam's education.
"She eventually wound up home schooling him because she battled with the school district," she said. "In what capacity I'm not 100 per cent certain.
"If it was her behaviour, if it was learning disabilities, I really don't know but he was a very, very bright boy. He was smart."
But she added that "it's the person that does the killing not the gun" and said she had found the incident "heartbreaking".
"You say 'it will never happen to me' but it has," she said. "I thank God every day my kids have faith, my kids know right from wrong - I'm not saying that hers didn't. You've got to give your kids roots."
Her comments came after Peter Lanza, the gunman's father, said his family was "struggling to make sense of what has transpired".
"Our hearts go out to the families and friends who lost loved ones and to all those who were injured," he said.
"Our family is grieving along with all those who have been affected by this enormous tragedy. No words can truly express how heartbroken we are.
"We are in a state of disbelief and trying to find whatever answers we can. We too are asking why. We have cooperated fully with law enforcement and will continue to do so."
Nancy and Peter Lanza divorced in 2009 after 28 years of marriage. The break up was traumatic, leaving the couple's sons devastated. Ryan Lanza was living away at university, meaning that his brother Adam, four years younger, was left at home alone with their mother at their £350,000 house.
He was not well known to neighbours, who describe him as being reclusive and troubled.
When the news broke on Friday of the murders, and Ryan Lanza was hastily identified as the killer, people who knew the family knew they had named the wrong brother.
"Adam Lanza has been a weird kid since we were five years old," said Tim Dalton, a neighbour and former classmate, on Twitter. "As horrible as this was, I can't say I am surprised."
"This was a deeply disturbed kid," a family insider said. "He certainly had major issues. He was subject to outbursts from what I recall."
A further family friend said he had acted as though he was immune to pain.
"A few years ago when he was on the baseball team, everyone had to be careful that he didn't fall because he could get hurt and not feel it," said the friend. "Adam had a lot of mental problems."
Lanza's brother Ryan reportedly told police that his sibling had autism or Asperger's syndrome, and a personality disorder.
He gave no details, but anti-social disorder - also known as sociopathy - is the type most closely linked with violence and criminal behaviour.
Studies have suggested that 50 per cent of the prison population meet the criteria for the diagnosis.
Those with such disorders are more likely to embark on impulsive, risk-seeking behaviour, in an attempt to escape feeling empty or emotionally void.
In such cases, they are likely to have little regard for the consequences of their actions, and are unlikely to experience fear.
Ryan also said that he had not seen him since 2010.
As the news was breaking, Ryan was at work in accountancy firm Ernst and Young, sitting at his desk in Times Square.
To his horror, the 24-year-old found that his name was flashing up on the television news networks, wrongly accused of the massacre. He fled the office, jumping on a bus to return home to the house he shared in New Jersey. Shaken, he told his neighbour in an online message that he thought his mother was dead and he knew who was responsible for the multiple murder.
"It was my brother," he said.
Those on the autistic spectrum have a more limited emotional range and can miss social cues, making it more difficult for them to communicate and feel empathy with others. Difficulties communicating can cause frustration, which can spill over into aggression.
Several studies have found that violence and criminal behaviour are no more common in those diagnosed with autism than they are in the general population.
Asperger's syndrome is a type of autism which is more commonly diagnosed in those with higher than average intelligence.
And Lanza was said by classmates to be fiercely intelligent.
"You could tell he was, I would say, a genius," said Miss Israel. "There was something that was above the rest of us."
He'd correct people's Latin homework, when they were aged around 14, and at 16 was among the list of top students in his English class, studying "Of Mice and Men" and "Catcher In The Rye" - the classic tale of troubled youth.
"It was almost painful to have a conversation with him, because he felt so uncomfortable," said Olivia DeVivo, who sat behind him in English. "I spent so much time in my English class wondering what he was thinking."
"He didn't have any friends, but he was a nice kid if you got to know him," said Kyle Kromberg, now studying business administration at Endicott College in Massachusetts. He studied Latin with Lanza.
"He didn't fit in with the other kids," he said. "He was very, very shy. He wouldn't look you in the eyes when he talked. He didn't really want to lock eyes with you for very long."
He was also a technical whizz kid, keen on computers and video games, and part of a group who would meet up for computer programming get-togethers.
"My brother has always been a nerd," Ryan said, according to Gloria Milas, whose son was a club member along with Adam Lanza.
Catherine Urso, who was attending a vigil on Friday evening in Newtown, said her college-age son knew the killer and remembered him for his alternative style.
"He just said he was very thin, very remote and was one of the goths," she said.
The siblings certainly carved out different paths in life.
Ryan went to university; followed his father into finance; was living with friends in an attractive red-brick property in New Jersey. Indeed, when the tragedy of Friday was unfolding, one of his housemates, Jessica O'Brien, wrote on Facebook: "Do you need anything ready for when you get home? Can I set anything out for you to grab and go? Anything else I can do?"
By contrast, Adam Lanza had few friends and, as a child, went to great trouble not to mix with his fellow students at his state school. A Newtown resident also suggested he was home-schooled for some time.
"I always saw him walking alone, sitting on his own at a table or on the bus. Most of the time I saw him he was alone," said Alex Israel, who was at school with him as a young girl.
"He was really quiet. A little fidgety, uneasy. I think socially he was just going out (into the world) and not making friends with everyone."
Her mother Beth Israel, who lived nearby, said: "I know he had issues. He was a really troubled kid ... a very quiet kid, a shy kid, maybe socially awkward." He was not on Facebook, unusually for any Westerner of his generation, and did not appear in his 2010 High School Yearbook. Instead were written the words: "Camera shy".
Forty miles away from Newtown, in the well-heeled Connecticut city of Stamford, Lanza's father Peter – who was divorced from the boys' mother Nancy – was returning home on Friday afternoon. A highly-qualified academic who a year ago was appointed vice president of taxes for energy investment firm GE Energy Financial Services, Mr Lanza wound down the window on his blue Mini Cooper and asked the person outside his home how he could help her.
"I explained that I'd been told someone at his address had been linked to the shootings in Newtown," said Maggie Gordon, a reporter from the local newspaper.
"His expression twisted from patient, to surprise, to horror."
Mr Lanza had moved out in 2009, remarrying a University of Connecticut librarian in January 2011. He was said to have last seen his son Adam in June. But the painfully shy young man had taken the divorce badly.
"The kids seemed really depressed" by the break-up, said Ryan Kraft, 25, who stayed with Adam when Mrs Lanza went out.
"He would have tantrums," Mr Kraft said. "They were much more than the average kid [had]."
Mr Lanza's lawyer Gary Oberst said: "He was very upset that he was getting divorced, but he didn't want to take it out on anybody.
"He did more than he had to with the divorce. When he came in to consult with me, I said 'This is what your obligation is.' And he said: 'That's not enough. I want to do more.'"
Mr Lanza agreed to pay $240,000 (£148,400) annually to his ex-wife, and Mrs Lanza appeared to live in comfort with Adam. There was also suggestions that she was unable to work.
"She needed to be home with Adam," one family insider said.
Marsha Lanza, aunt to the boys, described Mrs Lanza as a good mother and kind-hearted. Mrs Lanza would host games of dice, or else venture out to visit her neighbours for a glass of wine. The home was immaculate; the swimming pool behind the house well maintained.
But Mrs Lanza was also, according to friends, an avid gun collector.
Dan Holmes, owner of a Connecticut landscaping firm, said Mrs Lanza once showed him a "high-end rifle" that she had purchased, adding, "She said she would often go target shooting with her kids".
The gun used to shoot Mrs Lanza was her own.
Yet, perhaps predictably, the owner of the local rifle range was defiant.
Richard Dravis, who gives shooting training at Wooster Mountain rifle range, 15 miles away from the school, said: "We don't train crazy people. I think that if we would address the mental health issue here we could possibly do something in the future. But we can't count the number of rounds in the magazine of a nut head."
His grandmother was too distraught to speak when reached by phone at her home in Florida, Associated Press reported.
"I just don't know, and I can't make a comment right now," Dorothy Hanson, 78, said in a shaky voice as she started to cry.
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