NEW YORK (CBSNewYork/AP) -- In his most extensive comments, the father of Newtown gunman Adam Lanza said what his son did couldn't "get any more evil'' and he now wishes his son had never been born.
In a series of interviews last fall, Peter Lanza told The New Yorker magazine that 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," Lanza told the magazine.
Sandy Hook Shooter's Dad Wishes Son Had Never Been Born
He often contemplates what he could have done differently in his relationship with Adam, although he believes the killings couldn't have been predicted.
"Any variation on what I did and how my relationship was had to be good, because no outcome could be worse,'' Lanza 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.''
He said he hadn't seen his son in two years when Adam killed 20 first-graders and six educators at Sandy Hook Elementary School in December 2012.
Adam killed himself as police arrived. He also fatally shot his mother, Nancy, in their Newtown home before going to the school.
"The reason he shot Nancy four times was one for each of us: one for Nancy; one for him; one for (his brother) Ryan; one for me," Peter Lanza told the magazine.
He and Nancy Lanza separated in 2001 and divorced in 2009. He last saw Adam in October 2010 and wanted to maintain contact with him. But Nancy Lanza wrote him an email saying Adam didn't want to see him, despite her efforts to reason with him.
Peter Lanza said Adam was 13 when a psychiatrist diagnosed him with Asperger's syndrome, a form of autism not associated with violence. But he believes the syndrome "veiled a contaminant'' that wasn't Asperger's.
"I was thinking it could mask schizophrenia,'' he said.
He said 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. He 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.''
CBS News spoke with one couple who reached out to Lanza, Alissa and Robbie Parker. The couple's 6-year-old daughter, Emilie, was killed at Sandy Hook.
"I got the impression from him that he was grateful for the opportunity to offer his condolences and his apologies," Robbie Parker said.
One thing Peter Lanza did not reveal in the interview is what he did for Adam's funeral.
"No one knows that," he said. "And no one ever will."
A spokesman for Peter Lanza said Monday that Lanza would not be commenting further.
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