THE mother of US killer Adam Lanza took him shooting from the age of four and may have lied on paperwork to buy him guns, according to a damning new book.

Newtown: An American Tragedy, by US journalist Matthew Lysiak, also revealed Nancy Lanza would take long holidays without the boy she labeled a "lost cause", the Daily Mail reports.

The book comes year after the Newtown, Connecticut shooting - one of the worst in US history - which claimed the lives of 20 students and six teachers at Sandy Hook primary school.

But Lysiak suggests Adam, 20, may have initially targeted another school.

A source familiar with the investigation told the author Adam’s car was believed to have been spotted circling nearby Newtown High on the morning of the massacre.

"The official believed Adam spotted two police cars, which were parked in the lot, and decided to move on," the book says.

Newtown also depicts a killer years in the making, under the watch of a seemingly oblivious - and, towards the end, absent - mother.

Family friend Marvin LaFontaine told Lysiak he used to invite Nancy, Adam and his brother Ryan over for shooting sessions using a semi-automatic Ruger rifle.

Mr LaFontaine said Adam was "comfortable with a firearm" at the age of four.

"The Ruger was lightweight and easy to handle and, in Nancy’s opinion, the ideal weapon for her young child," he is quoted as saying in the book.

When Adam began having problems at school, Nancy reportedly saw shooting as the "perfect outlet" for "bonding sessions" together.

In order to do allow those activities, Nancy allegedly lied to buy Adam guns.

Official records show Nancy obtained weapons - including that of the Bushmaster XM15 rifle and a SIG Sauer 9-millimetre pistol used in the school shooting - by indicating she was purchasing them for herself and nobody else.

"Some of the weapons Nancy bought she kept. Others she gave to Adam," the book said.

Nancy, who raised Adam on her own after her divorce from his father, Peter, told friends for years that she thought there was "something very wrong" with the future school shooter, and that he was a "lost cause".

In the months before the December 14 attack, she began taking regular trips - to London, Boston, New York, San Francisco and New Orleans - leaving Adam at home alone and telling a friend it was "already too late" to help him.

Three days before her son’s massacre, Nancy checked into an out-of-state spa resort, where she messaged a friend that she was having a "heavenly" time.

At home, the book said, Adam had stopped taking his medication for Asberger’s and would play violent video games alone for days on end, in a room with blacked out windows and littered with newspaper cuttings relating to mass shootings.

Adam shot and killed Nancy at their Newtown home on the morning of the Sandy Hook massacre, after which he turned a gun on himself.

Peter Lanza has not made any public comments beyond his first statement after the attack.

"Our family is grieving along with all those who have been affected by this enormous traged," he said at the time.

"No words can truly express how heartbroken we are. We are in a state of disbelief and trying to find whatever answers we can."

Originally published as Lanza's mum 'took him shooting at 4'