The father of Sandy Hook school gunman Adam Lanza has broken his silence on the tragedy and said he wished his son had never been born.

"That's totally where I am," Peter Lanza told New Yorker magazine in an interview about his son's gun rampage at Sandy Hook Elementary school in Newtown, Connecticut, in December 2012.

Lanza said: "You can't get more evil."

His son shot dead 26 children and teachers at the school after killing his own mother.

Peter Lanza said: "No outcome could be worse. How much do I beat up on myself about the fact that he's my son? A lot."

He said his wish that his son had never been born was not natural but added: "But, God, there's no question. There can only be one conclusion, when you finally get there."

The shooting of 20 children and six members of staff at the school reignited the debate on guns in the United States.

Adam Lanza took his own life as police officers arrived on the scene. Investigators later discovered that he had detailed records about other mass murders on a 7ft x 4ft spreadsheet. The document was so big that special equipment was required to print it.

Peter Lanza had separated from his son's mother in 2001 and they were divorced in 2009. Adam Lanza did not see his father in the two years leading up to the fatal shootings.

Peter said he was worried about his son's behaviour as long as seven years ago when he started middle school.

Lanza said: "It was crystal clear something was wrong. 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.

"Quite honestly, I think that I wouldn't recognise the person I saw," he added.

"All I could picture is there'd be nothing there, there'd be nothing. Almost, like, 'Who are you, stranger?'"