A spokesman for Peter Lanza said Monday that Lanza would not be commenting further.
Peter Lanza told the magazine that his son as a young child "just a normal little weird kid."
But as he grew older, Adam's mental health problems worsened, according to Connecticut State Police documents. A Yale University professor diagnosed Lanza in 2006 with profound autism spectrum disorder, "with rigidity, isolation, and a lack of comprehension of ordinary social interaction and communications," while also displaying symptoms of obsessive-compulsive disorder, the documents show.
Peter Lanza said his and Nancy Lanza's concerns about Adam increased when he began middle school.
"It was crystal clear something was wrong," he said. "The social awkwardness, 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."
After the killings, police investigators discovered that Adam Lanza had written violent stories as a child and later became interested in mass murders.
Peter Lanza believes his son had no affection for him at the time of the shootings.
"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 Lanza was asked how he would feel if he could see his son again.
"Quite honestly, I think that I wouldn't recognize the person I saw," he said. "All I could picture is there'd be nothing there, there'd be nothing. Almost, like, 'Who are you, stranger?'"
He said he wished Adam had never been born.
"That didn't come right away," Peter Lanza said about that statement. "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."