He carried a black briefcase to his 10th-grade honors 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, 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 Lanza, 20, wearing combat gear, carried out one of the deadliest mass slayings in the nation's history. He killed his mother, a kindergarten teacher, at home and 20 children at the elementary school where she worked. He also killed six adults at the school, before apparently turning his gun on himself.

In his brief adulthood, 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 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 to be 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."

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

His brother told police that Adam was "somewhat autistic," according to an Associated Press report.

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

Others said 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?"'

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

Still, after hearing of the news on Friday, 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."

Baier, who sat next to Lanza in the back of their sophomore-year honors 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.

Out of view of his classmates, Lanza's adolescence seemed to have been turbulent. In 2006, his older brother graduated high school and went to Quinnipiac University in Connecticut, leaving him alone with their parents - whose marriage was apparently coming apart. 

In 2008, they divorced, after 17 years, court records show. His father, Peter Lanza, a tax executive for General Electric, moved to Stamford, and married in January 2011 a woman who is a librarian at the University of Connecticut.

His mother, Nancy Lanza, kept their home in Newtown, in a prosperous, hilly enclave of spacious, newer homes about five miles from the elementary school where she taught kindergarten. Adam Lanza is thought to have been living in the house, too.

Friends remembered Nancy Lanza as very involved in her sons' lives.

"Their mother was very protective, very hands-on," said Gina McDade, whose son was a playmate of Ryan Lanza's and spent much time at his home, which she described as a two-story Colonial with a pool.

"It was a beautiful home," McDade said. "She was a good housekeeper, better than me. You could tell her kids really came first."

On Friday, police officers and agents from the FBI swarmed through the Lanzas' neighborhood, blocking off streets and asking residents to leave their homes.

Throughout the afternoon, Nancy Lanza's surviving son, Ryan, was named by some news outlets as the killer.

Ryan Lanza's identification had been found on the body of his brother, leading to the mistaken reports.

Brett Willshe, a neighbor of Ryan Lanza's in Hoboken, said he communicated with him by instant message at 1:15 p.m.

"He said he thought his mom was dead, and he was heading back up to Connecticut," Willshe said. "He said, `It was my brother."'