Parents embrace after the shooting at the Sandy Hook Elementary School in Newtown, Connecticut. (AFP)

Dec. 15: He carried a black briefcase to his 10th-grade honours English class, and sat near the door so he could readily slip in and out. When called upon, he was intelligent but nervous and fidgety, spitting his words out as if having to speak up were painful.

Pale, tall and scrawny, Adam Lanza walked through high school in Newtown, Connecticut, with his hands glued to his sides, the pens in the pocket of his short-sleeve, button-down shirts among the few things that his classmates recalled about him.

He did all he could to avoid attention, it seemed. Until yesterday. The authorities said Lanza, 20, wearing combat gear, carried out one of the deadliest school shootings in American history.

He killed 20 children and six adults at the elementary school where his mother worked, they said. He then apparently turned his gun on himself. Earlier, he had killed his mother at their home, the police said.

In his brief adulthood, Lanza had left few footprints, electronic or otherwise. He apparently had no Facebook page, unlike his older brother Ryan, 24, a Hoboken, New Jersey, resident who for several hours yesterday was misidentified in news reports as the perpetrator of the massacre.

Adam Lanza did not even appear in his high school yearbook, that of the class of 2010. His spot on the page said: “Camera shy”. Others who graduated that year said they did not believe he had finished school.

Matt Baier and other school classmates recalled how deeply uncomfortable Lanza was in social situations.

Several said in separate interviews that they understood that he had a developmental disorder, Asperger’s syndrome, considered a high-functioning form of autism.

“It’s not like people picked on him for it,” Baier said. “From what I saw, people just let him be, and that was that.”

Police officers said they were examining whether Lanza had such a disorder.

A former classmate who said he was familiar with the disorder described Lanza as having a “very flat affect”, adding: “If you looked at him, you couldn’t see any emotions going through his head.”

Classmates said Lanza’s evident discomfort prompted giggles from those who did not understand him.

“Maybe he wasn’t given the right kind of attention or help,” Olivia DeVivo said. “I think he went so unnoticed that people didn’t even stop to realise that maybe there’s actually something else going on here — that maybe he needs to be talking or getting some kind of mental help”

DeVivo remembered Lanza from sixth grade and earlier, talking about aliens and “blowing things up”, but she chalked this up to the typical talk of pre-pubescent boys.

After hearing the news yesterday, DeVivo reconnected with friends from Newtown, and the consensus was stark.

“They weren’t surprised,” she said. “They said he always seemed like someone capable of that because he just didn’t really connect with our high school, and didn’t really connect with our town.”

She added: “I never saw him with anyone. I can’t even think of one person that was associated with him.” Adam Lanza, however, belonged to a school technology club that held “LAN (local area network) parties” in which students would gather at a member’s home, hook up their computers into a small network and play games.

Joshua Milas, a fellow club member, said Adam Lanza was “probably one of the smartest kids I know; he was probably a genius”.

Baier, who sat next to Lanza in their sophomore-year honours math class, said Lanza barely said a word all year but earned high marks. He said he knew this only from peeking at Lanza’s scores when their teacher handed back their tests.

Lanza’s adolescence seemed to have been turbulent. In 2006, Ryan graduated from high school and went to university in Connecticut, leaving him alone with their parents, whose marriage was apparently coming apart. In 2008, they divorced after 17 years. Peter Lanza, a tax executive for General Electric, moved to Stamford and re-married.

Lanza’s mother Nancy kept their two-storey home in Newtown, a prosperous enclave about 95km from New York, where neighbours are doctors or hold white-collar positions at companies such as General Electric, Pepsi and IBM. Adam Lanza was thought to have been living in the house, which has a pool.

“Their mother was very protective, very hands-on,” said neighbour Gina McDade. “It was a beautiful home. She was a good housekeeper, better than me. You could tell her kids really came first.”

Some news outlets had named Ryan as the killer after his identification was found on Adam Lanza’s body.

Ryan was questioned but an official said he was not believed to have had a role in the rampage. He told investigators he had not been in touch with his brother since about 2010.

Brett Wilshe, a neighbour of Ryan’s in Hoboken, said he communicated with him by instant message at 1.15pm.

“He said he thought his mom was dead, and he was heading back up to Connecticut,” Wilshe said. “He said, ‘It was my brother’.”