Adam Lanza seemed not to feel physical or psychological pain in the same way as his classmates at Newtown High School, a former employee there has said.
Richard Novia, the school district's head of security until 2008, who also served as adviser for the school technology club, said Lanza clearly "had some disabilities".
Mr Novia was responsible for monitoring students as they used soldering tools and other potentially dangerous electrical equipment.
He recalled meeting school guidance counsellors, administrators and the boy's mother Nancy to understand his problems and find ways to ensure his safety. But there were other crises only a mother could solve.
"He would have an episode, and she'd have to return or come to the high school and deal with it," Mr Novia said, describing how the young man would sometimes withdraw completely "from whatever he was supposed to be doing", whether it was sitting in class or reading a book.
Lanza "could take flight, which I think was the big issue, and it wasn't a rebellious or defiant thing", Novia said. "It was withdrawal."
Authorities are trying to understand what led the young man to kill his mother in their home and then slaughter 26 children and adults at a Connecticut elementary school before taking his own life.
A law enforcement source said Lanza was diagnosed with Asperger's, a mild form of autism. People with the disorder tend to function poorly socially but can be highly intelligent.
If he did have Asperger's, his lack of sensation could be related to the disorder, said psychologist Elizabeth Laugeson, an assistant clinical professor at the University of California, Los Angeles. People with Asperger's can be overly sensitive to things like touch, noise and pain, or sometimes under-sensitive, she said.
In their teenage years, Lanza and his older brother, Ryan, were members of the tech club, which offered students a chance to work on computers, videotape school events and produce public access broadcasts.
It was popular among socially awkward students, but Lanza, while clearly smart, had problems that went beyond an adolescent lack of social skills, Mr Novia said.
"You had yourself a very scared young boy, who was very nervous around people," Mr Novia said.
The club provided a setting for students to build lasting friendships. But while other members were acquainted with Lanza, none was close to him.
He was not physically bullied, although he might have been teased, Mr Novia said.
The club gave the boy a place where he could be more at ease and indulge his interest in computers. His anxieties appeared to ease, but they never disappeared. When people approached him in the hallways, he would press himself against the wall or walk in a different direction, clutching tight to his black case.
"The behaviour would be more like an eight-year-old who refuses to give up his teddy bear," Mr Novia said. "What you knew with Adam is, it was a possession. It was not a possession to be put at risk."
Even so, Mr Novia said, his primary concern was that Lanza might become a target for abuse by his fellow students, not that he might become a threat.
"Somewhere along in the last four years, there were significant changes that led to what has happened," Mr Novia said. "I could never have foreseen him doing that."
Jim McDade, who lives a few houses from where Ms Lanza was killed, said his family got to know the two brothers and their mother because their children were about the same ages and rode the school bus together.
"There was certainly no indication of anything unusual that lets you think that a kid's going to do something like that," said Mr McDade, who works in finance in New York. "There was nothing that would indicate anything going on behind the scenes that would lead to this horrible mess."
He recalled Lanza as "a very bright kid".
Olivia DeVivo, a student at the University of Connecticut, was in Lanza's 10th grade English class.
"He was very different and very shy and didn't make an effort to interact with anybody," she said.
Ms DeVivo said Lanza always carried a briefcase and wore his shirts buttoned up to the top. She said he seemed bright but never really participated in class.
"Now, looking back, it's kind of like 'okay, he had all these signs', but you can't say every shy person would do something like this."