Adam Lanza killed himself after killing 27 others (Screenshot ABC News)

As the world mourns the Sandy Hook Elementary School shooting, one of the worst massacres in the nation's history, questions continue to remain unanswered as to what prompted the gunman, 20-year-old Adam Lanza, to go on a killing spree.

Details about Lanza– who killed himself after shooting his mother and 26 others at the school, including 20 children– have begun to surface. Described as someone who often kept to himself, he also seemed unaffected by pain, reported The Associated Press.

According to Richard Novia, the Newtown High School adviser for the school technology club and the school district's head of security until 2008, Lanza "had some disabilities."

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• "If that boy would've burned himself, he would not have known it or felt it physically," Novia told The Associated Press in a phone interview. "It was my job to pay close attention to that."
• Novia was responsible for monitoring students as they used soldering tools and other potentially dangerous electrical equipment.
• Novia recalled meeting with school guidance counselors, administrators and with the boy's mother, Nancy Lanza, to understand his problems and find ways to ensure his safety. But there were other crises only a mother could solve.
• "He would have an episode, and she'd have to return or come to the high school and deal with it," Novia said, describing how the young man would sometimes withdraw completely "from whatever he was supposed to be doing," whether it was sitting in class or reading a book.
• Adam Lanza "could take flight, which I think was the big issue, and it wasn't a rebellious or defiant thing," Novia said. "It was withdrawal."

Lanza's former classmates described him in a similar manner and they had been told he had Asperger's syndrome, The New York Times reported. According to the AP, the disorder could have caused a lack of sensitivity as people with Asperger's can be "overly sensitive to things like touch, noise and pain, or sometimes under sensitive."

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• “He was always different — keeping to himself, fidgeting and very quiet,” said a classmate, Alex Israel. “But I could always tell he was a supersmart kid, maybe just socially awkward, something just off about him. The same went for when I went to his house. His mother was always nice to me; she was a kind, typical suburban mom as far as I remember. As time went on, he continued to keep to himself and I branched out more, so not much contact with him after middle school.
• “By the time high school came around, he did sort of disappear,” she added. “I’d see him in the halls walking quickly with his briefcase he carried, but I never had a class with him and never saw him with friends. I was yearbook editor and I remember he declined to be photographed or give us a senior quote or baby picture.”

Lanza's first victim had been his mother, Nancy Lanza, whom he shot with a gun from her own gun collection before driving to Sandy Hook Elementary School in Newtown, Conn.

"Investigators have linked Ms. Lanza to five weapons: two powerful handguns, two traditional hunting rifles and a semiautomatic rifle that is similar to weapons used by troops in Afghanistan. Her son took the two handguns and the semiautomatic rifle to the school. Law enforcement officials said they believed the guns were acquired legally and were registered," according to The New York Times.

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Nancy Lanza, 52, often took her sons to one of the shooting ranges in the suburbs northeast of New York, according to friends of hers. A former stockbroker at John Hancock, she had filed for divorce in 2008 and was described by friends as "social and generous to strangers but also high-strung as if she were holding herself together… She lived in a large Colonial home here with Adam Lanza, and had struggled to help him cope with a developmental disorder that often left him reserved and withdrawn, according to relatives, friends and former classmates," reported The New York Times.

Initial reports said that Nancy Lanza worked at the elementary school, but on Saturday the school superintendent said there was no record of her ever working there; and authorities said it's not clear why Lanza went to the school, said The Times.

The Governor of Connecticut said on Sunday that Adam Lanza shot himself as first responders closed in on him and suggested that Lanza "had planned an even more gruesome massacre and was stopped short," according to ABC News.

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