NEW YORK (AFP) ― No one knows yet what transformed nerdy, small town loner Adam Lanza into a black-clad monster who gunned down 26 people, including 20 young children, at an elementary school.

What many acquaintances agree on is that Lanza, 20, was as quiet as the Connecticut community of Newtown where he unleashed his massacre at Sandy Hook Elementary School on Friday.

Other than a 2008 divorce filing between his parents, his background suggests a stable and well-to-do family.

He reportedly lived with his mother Nancy Lanza in the suburban town, where she may have previously worked with the elementary school. Her big, detached house is in affluent neighborhood of the leafy town.

His tax specialist father is an executive at GE Capital, and his 24-year-old brother Ryan, who was initially confused for the killer, also works in tax practice.

But the scrawny youngster seen in a rare photo circulating online was a nerdy loner with little ability to fit in.

“He was a weird kid as a child, but he was just quiet. We never knew his real personality. He was just himself,” said former classmate Megan, 20.

“He fell off the radar half-way through middle school, maybe seven or eight years ago,” she told AFP.

The New York Times quoted friends as saying that Adam Lanza was intelligent “but nervous and fidgety.”

At school, “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.”

And when he walked, “his hands glued to his sides,” his body language was that of someone who wanted to be invisible, the Times said. “He did all he could to avoid attention, it seemed.”

Lanza also shrank from social media, in contrast to many American youths.

No Facebook page has been found so far. In his high school yearbook, for the class of 2010, his photo is missing. “Camera shy,” the empty space says, reported the Times.

A troubled soul, as well as possible health problems, loomed behind that quiet facade. Ryan Lanza told police his brother had a history of mental issues, CBS reported.

Former neighbor Ryan Kraft told the Washington Post that the Lanza brothers were upset by their parents’ breakup and that Adam had obvious problems.

“He would have tantrums,” Kraft told the Post. “They were much more than the average kid (would have).”

Alex Israel, a former classmate, told CNN that Lanza was “fidgety” and “a little uneasy if you were to look at him.”

“Socially (he was) not as ready to go out there and make friends,” she said. “He preferred just to stay by himself.”

A relative told ABC News that Adam was “obviously not well.”

Cruelly, it seems that his murderous plan was enabled inadvertently by his own mother Nancy.

The powerful Sig Sauer and Glock handguns he used -- types commonly issued to law enforcement officers -- as well as a military-grade rifle, were registered to her.

There has been no explanation yet as to why a suburban mom in a genteel neighborhood would have wanted such lethal arms.

Police have indicated that his mother was Adam Lanza’s first victim.

After shooting her in the face in her own house, with her own gun, he then seems to have driven to the school and methodically spread mayhem, before taking his own life.

The question why remains unanswered.

”We’ll probably never know for sure,“ Tom Fuentes, a former FBI assistant director, said.

”We don’t know what triggered him to actually (carry) this out and unfortunately he’s not available to ask,“ he told CNN. “His mother should certainly know him best and she’s not available.”