Peter Lanza spoke about his son's childhood and how he reacted after he found out his son was behind the massacre in an article dated March 17.
He says he called his now wife and told her over and over: "I think it's Adam. It's Adam."
Peter says he carries guilt about the incident, despite not seeing his son in the two years prior to the rampage.
He told police that Adam had Asperger's syndrome - a type of autism that is not associated with violence - and exhibited symptoms of being "slightly OCD," meaning obsessive compulsive disorder.
He told the New Yorker Magazine that he often contemplates what he could have done differently to improve his relationship with Adam, but believes the killings couldn't have been prevented.
"Any variation on what I did and how my relationship was had to be good, because no outcome could be worse," Peter said. "You can't get any more evil ... How much do I beat up on myself about the fact that he's my son? A lot."
Peter said Adam was "just a normal little weird kid," but his concerns about Adam increased when he began middle school.
"It was crystal clear something was wrong," he said. "The social akwardness, the uncomfortable anxiety, unable to sleep, stress, unable to concentrate, having a hard time learning, the awkward walk, reduced eye contact. You could see the changes occurring."
On Dec. 14, 2012, Adam killed his mother Nancy, and then drove to the school where he carried out his rampage. The shooting claimed the lives of six adults and 20 children. He killed himself as police arrived.
Peter told the New Yorker Magazine that he has met with two families of his sons' victims and didn't know how to react when they offered their forgiveness.
"A victim's family member told me that they forgave Adam after we spent three hours talking. I didn't even know how to respond," he said.
Peter says he's also received letters of support from across the country, but avoided eating some of the candy included in the letters, in case it contained poison.
Nearly 15 months after the shooting, Peter says that he is haunted by his son and dreams of him nightly. He says he believes Adam would have killed him, too, if he had the chance.
"With hindsight, I know Adam would have killed me in a heartbeat, if he'd had the chance. I don't question that for a minute," he told the magazine.
Peter and Nancy separated in 2001 and divorced in 2009. He last saw Adam in October 2010 and allegedly wanted to mantain contact with him, but Adam ignored his father's requests. He now says he wishes Adam had never been born.
"That didn't come right away," Peter said. "That's not a natural thing, when you're thinking about your kid. But, God, there's no question. There can only be one conclusion, when you finally get there. That's fairly recent, too, but that's totally where I am."
ABC News and The Associated Press contributed to this report.