Dec. 15, 2012 — -- Adam Lanza, the 20-year-old who killed 20 kids and six adults at a Connecticut elementary school Friday, was very bright, say neighbors and former classmates, but he was also socially awkward and deeply troubled.
"[Adam] was not connected with the other kids," said family friend Barbara Frey. A relative told ABC News that Adam was "obviously not well."
READ full ABC News coverage of the Connecticut shootings.
On Friday morning, Lanza shot his mother Nancy in the face at the home they shared in Newtown, and then drove her car to Sandy Hook Elementary School. Dressed in black combat gear, he broke a window at the school, which had recently had a new security system installed, and within minutes had shot and killed six adults and 20 schoolchildren between the ages of five and 10.
The shooting stopped when Lanza put a bullet in his own head. Multiple weapons were found at the scene, including two semiautomatic handguns registered to his mother. A Bushmaster rifle registered to Nancy was discovered outside in the car.
Police are currently investigating reports that Lanza had an altercation at the school in the days before the shooting, according to sources.
Long before Lanza's spree, however, residents of Newtown had noticed that tall, pale boy was different, and believed he had some kind of unspecified personality disorder.
"Adam Lanza has been a weird kid since we were five years old," wrote a neighbor and former classmate Timothy Dalton on Twitter. "As horrible as this was, I can't say I am surprised."
In school, Lanza carried a black briefcase and spoke little. Every day, he wore a sort of uniform: khakis and a shirt buttoned up to the neck, with pens lined up in his shirt pocket.
A former classmate in his 10th grade honors English class, Olivia DeVivo, says he "was always very nervous and socially awkward."
She told ABC News that "he didn't really want to be spoken to" and that when teachers would call on him "it appeared physically difficult for him to speak."
Lanza avoided public attention and had few, if any, friends, though he was a member of the high-school tech club. He liked to sit near the door of the classroom to make a quick exit.
He even managed to avoid having his picture in his seventh-grade yearbook. He was one of seven students listed as "Camera Shy." And unlike most in his age group, he seems to have left little imprint on the internet – no Facebook page, no Twitter account.
Lanza's parents Peter and Nancy Lanza married in New Hampshire in 1981, and had two sons, Adam and his older brother Ryan, who is now 24 and lives in New Jersey.
The Lanzas divorced in 2009 after 28 years of marriage due to "irreconcilable differences." When they first filed for divorce in 2008, a judge ordered that they participate in a "parenting education program."
Adam was 17 at the time of the divorce. He continued to live in Newtown with his mother. His father now lives in his Stamford, Connecticut with his second wife.
Peter Lanza, who drove to northern New Jersey to talk to police and the FBI, is a vice president at GE Capital and had been a partner at global accounting giant Ernst & Young.
Adam's older brother Ryan Lanza, 24, has worked at Ernst & Young for four years, apparently following in his father's footsteps and carving out a solid niche in the tax practice. He too was interviewed by the FBI. Neither he nor his father is under any suspicion.
"[Ryan] is a tax guy and he is clean as a whistle," a source familiar with his work said.
Police had initially identified Ryan as the killer. Ryan sent out a series of Facebook posts saying it wasn't him and that he was at work all day. Video records as well as card swipes at Ernst & Young verified his statement that he had been at the office.
Two federal sources told ABC News that identification belonging to Ryan Lanza was found at the scene of the mass shooting. They say that identification may have led to the confusion by authorities during the first hours after the shooting.