He was not well known to neighbors, who describe him as being reclusive and troubled.
And when the news broke on Friday of the murder of 26 people at a primary school in the town, 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.