Former schools security chief Richard Novia, who knew the Newtown gunman, Adam Lanza, reveals some details about the killer's life ITN

This weekend the name of Adam Peter Lanza was added to what is becoming a distinctly American list – a catalogue of young men who shot their way from small town obscurity to infamy by carrying out the mass killing of other young people.

In the desperate scramble to name the perpetrator who forced his way into a Connecticut school on Friday and shot dead 20 children, the family and friends of the Lanza family were rooted out through their social media pages and old-fashioned door knocking, with even 20-year-old Lanza's father, Peter, hearing about the shootings from a local paper reporter as he drove home.

At first police named Ryan Lanza, 24, Adam's brother, as the shooter, leading to a flurry of denials by him on social networking sites as he left his job at Ernst & Young in Times Square, and headed to the New Jersey house he shares with two friends. Ryan, who left the family home for university in 2006, wrote on Facebook: "Fuck you CNN it wasn't me." He posted: "I'm on the bus home now it wasn't me." His panicking friends were deluged by calls and messages.

Ryan Lanza has been helping police with their investigation and US media reported that he told them he had not seen his brother since 2010. But it became clear it was Adam Lanza who had shot his mother, Nancy, at the home they shared on Friday morning, then driven her car to the Sandy Hook elementary school five miles away. He forced his way in and shot dead 20 children, six adults and then himself.

The law enforcement official said investigators were unable to establish any connection so far between Nancy Lanza and the school.

Lanza's place in the high school year book of Newtown high for 2010 held no photograph, only the words "camera shy".

An honours student, he was a thin, awkward but bright boy who seemed socially uncomfortable. With retrospect some of his then classmates have said perhaps he had autism or Asperger's syndrome – that is what has been reported in local media to be what his brother has told the police. Lanza had no criminal record and no history of his causing trouble in the past, either at school or in his affluent neighbourhood of Newtown, some 90 miles from New York City. Some of his former classmates had trouble remembering anything about him at all.

Joshua Milas, who graduated from Newtown high school in 2009, said Adam Lanza was generally a happy person but that he had not seen him in a few years.

"We would hang out, and he was a good kid. He was smart," Milas said. "He was probably one of the smartest kids I know. He was probably a genius."

They talk of a boy who dressed smartly and worked hard, but who barely said a word during his time at school and made few friends. Intelligent but shy and nervous, most said. A former classmate, Olivia DeVivo, told the New York Times: "I never saw him with anyone. I can't even think of one person that was associated with him."

He had no Facebook page and his electronic footprint was minimal although yesterday the police chief seemed to suggest he may have left behind emails which could help explain his state of mind.

Nancy and Peter Lanza, a tax director for General Electric, separated in 2006, divorced in 2009 and Peter remarried in 2011. A neighbour who claimed to have babysat for Adam said the divorce had "hit him hard".

State police records show Nancy Lanza had bought five guns. Yesterday police said they were still investigating the origins of weapons used by her son.

Nancy's sister-in-law, Marsha Lanza, told AP her nephew had been raised by kind, nurturing parents, who would not have hesitated to seek counselling for their son if he needed it.

She was close to Lanza's mother whom she described as a "good mother and kind-hearted". Marsha Lanza said her husband saw Adam as recently as June and recalled nothing out of the ordinary about him.

• This article was amended on 19 December to make it clear that Marsha Lanza was sister-in-law to Nancy Lanza, not sister.