Pale, tall and scrawny, Adam Lanza walked through high school in Newtown, Conn., 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 friday
The authorities said Mr. Lanza, 20, wearing combat gear, carried out one of the deadliest school shootings in the nation’s history. He killed 20 children and six adults at the elementary school, they said. He then apparently turned his gun on himself. Earlier, the police said, he also killed his mother

In his brief adulthood, Mr. Lanza had left few footprints, electronic or otherwise. He apparently had no Facebook page, unlike his older brother, Ryan, a Hoboken, N.J., resident who for several hours on Friday 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, now a junior at the University of Connecticut, and other high school classmates recalled how deeply uncomfortable Mr. Lanza was in social situations

Several said in separate interviews that it was their understanding that he had a developmental disorder. They said they had been told that the disorder was Asperger’s syndrome, which is considered a high functioning form of autism

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

Law enforcement officials said Friday that they were closely examining whether Mr. Lanza had such a disorder

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

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

"You could tell that he felt so uncomfortable about being put on the spot,” said Olivia DeVivo, also now at the University of Connecticut. "I think that maybe he wasn’t given the right kind of attention or help. I think he went so unnoticed that people didn’t even stop to realize that maybe there’s actually something else going on here — that maybe he needs to be talking or getting some kind of mental help. In high school, no one really takes the time to look and think, ‘Why is he acting this way?’ ”

Ms. DeVivo remembered Mr. Lanza from sixth grade and earlier, talking about aliens and "blowing things up,” but she chalked this up to the typical talk of prepubescent boys.

Still, after hearing of the news on Friday, Ms. DeVivo reconnected with friends from Newtown, and the consensus was stark. "They weren’t surprised,” she said. "They said he always seemed like he was someone who was 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.”

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