Acquaintances of the family drew a sharper picture of Lanza, 20, as investigators attempt to retrace his path on December 14 from the 370 square metre home where he shot his mother multiple times to a nearby elementary school. There, he shot dead 20 students and six staff and then killed himself.

When Lanza would have one of his "episodes", Novia said, he would telephone Nancy Lanza. She was "a great parent", he said, and would often come within minutes, sitting with her son and making him feel better.

"She could pull him back in line," Novia said.

In high school, Adam Lanza always wore the same green shirt and khaki pants every day and hardly ever talked to classmates. He once gave a presentation entirely by computer, never uttering a single word, just demonstrating the steps on a screen.

"As long as I knew him, he never really spoke," said Daniel Frost, who took a computer class with Lanza and remembered his skill with electronics.

Someone in the class brought in the video game called , a first-person shooting game in which players compete against each other as either terrorists or counterterrorists, Frost said. Lanza "seemed pretty interested in the game", Frost recalled, and would play it with other students. He remembers the weapons Lanza chose: an M4 military-style assault rifle and a Glock handgun.

Lanza appears to have left high school early, and at age 16 began taking classes at Western Connecticut State University in nearby Danbury, where he earned a B-plus average.

Starting university at 16 would have been jarring, Novia said, especially as Lanza's older brother left for college and later for a job in Manhattan and their parents separated, leaving Lanza at home with his mother. The couple divorced in 2009.

Lanza's older brother, Ryan Lanza, now 24, was also a member of the tech club.

"There was a clear disconnect. Ryan was outgoing, energetic, well respected, recognised for his talents," Novia said. Ryan took care of his brother, but Novia said he heard they had become estranged in recent years.

The basement of the Lanza home was fully carpeted and had artwork, including a picture of a horse, on the walls. There was a computer, a flat-screen television, couches and an elaborate set-up for video games.

Nancy Lanza kept her guns in what appeared to be a secure case in another part of the basement, said Russell Ford, a friend of Nancy who had done chimney and pipe work on her home.

Lanza seemed to spend most of his time in his own large space in the basement, Ford said.

Investigators have not managed to retrieve any data from a computer they took from Lanza's house. "It looked like he took steps to damage it - he smashed it," said an official.

The Federal Bureau of Investigation has joined Connecticut state police in an unsuccessful effort to recover data that might lead to some understanding of what prompted Lanza to go on a rampage.

Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives have determined that Lanza and his mother had visited firing ranges, sometimes together, sometimes separately.

Friends said Nancy Lanza was no wild-eyed gun nut, but a gentle and caring woman who happened to love shooting.

It appears that after his parents' divorce, Adam Lanza broke off relations both with his father and his elder brother. The terms of the divorce indicated that Lanza would "reside primarily with the mother".

Nancy Lanza struggled to take care of her son and live a life of her own, friends said.

John Bergquist, who got to know Nancy Lanza at a neighbourhood bar where both were regulars, said she had a soft side when it came to her younger son.

"She always spoke very lovingly about him. She was devoted to him, catering to him and his limitations," he said.

"He wasn't troubled or violent in any way - he was a normal kid with a disability … He had trouble being with people."

Nancy Lanza appeared to have made the decision to move so that Adam could attend college or university in another state.

"She was willing to uproot her life," Bergquist said.

"Nancy pretty much made it clear that she needed to be with him because he couldn't handle being on his own."

But a Fox News report suggested Adam Lanza may have been motivated by anger at his mother because of plans to have him committed for treatment, citing comments from local sources.

It also cited an unnamed senior law-enforcement official saying anger at plans for "his future mental-health treatment" were being investigated as a possible motive.

Ford said Nancy Lanza had told him that she planned to move out west and enrol Adam in a "school or a centre". The plan started unfolding after Adam turned 18. "He wouldn't be dwelling with her," Ford said.

"She knew she needed to be near him," he added. "She was trying to do what was positive for him."

Ford said Nancy Lanza did not elaborate on what type of services she wanted her son to receive.

At Western Connecticut, few remember Lanza at all. German professor Renate Ludanyi said her records showed she gave him a "repeat" at the end of his time there - just short of failing.

"I must have had some hope that he was smart and could do it again, instead of really flunking him," she said.

"Probably he didn't come to class very often," she said. "You get so many students, they come and go. Some speak with you, some are outstanding, some … you barely remember when the semester's over."

Lanza fitted into that last category, she said. "He never talked to me. He came in, sat down, and left. He was just there."