Adam Lanza, the 20-year-old who killed children and adults at Sandy Hook Elementary School Friday, was bright but deeply troubled and had difficulty socializing with others, according to sources.

A former classmate in his 10th grade honors English class, Olivia DeVivo, said Adam "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."

CBS News' Scott Pelley reported on "60 Minutes" Sunday that friends of Adam's mother, Nancy, said that the mother called her son brilliant but disabled. She revealed Adam suffered from Asperger's Syndrome, a form of autism, which he was also on medication for. Children and adults with Asperger's have a normal range of intellectual capacity but have an "impairment" in social interaction and communication skills, and they have restrictive interests such as a preference for routine and consistency and development of special interests, according to aspergersyndrome.org.

When Pelley interviewed DeVivo and asked her to describe Adam socially, she said he never "put himself out there" and "it was just he wanted to be left alone and we left him alone."

Adam's aunt, Marsha Lanza of Chicago, said her nephew was never a violent child but she knew he had "issues."

"He really was a computer geek,'' she added. "He was a very bright boy."

ABC also noted that Adam's neighbor and former classmate, Timothy Dalton, wrote on his Twitter account "Adam Lanza has been a weird kid since we were five years old. As horrible as this was, I can't say I am surprised."

Nancy and her husband Peter divorced years back. They also have an older son named Ryan.

Funerals for victims of the Sandy Hook Elementary School shooting are already underway and President Obama attended a service on Sunday for the 20 children and plus adults slayed in the attack.