Adam Lanza planned Sandy Hook massacre after mother left him at home alone

Killer Adam Lanza plotted America’s worst school shooting after his mother left him alone for three days and went on holiday, it emerged today.

Gun collector Nancy Lanza, 54, had just returned when her disturbed son snapped and shot her dead before murdering 20 children and six staff at the local primary school.

Officers believe Lanza, 20, decided to carry out his massacre at Sandy Hook elementary school as he brooded alone beside his mother’s arsenal.

Ex-classmates revealed that the introverted loner worshipped the occult and had an online page dedicated to Satan, with “Devil” in red Gothic letters on a black background.

The FBI is trying to piece together Lanza’s smashed computer hard drive to establish a motive for the killings.

Officers are thought to believe he used devil-worship and suicide sites — and even boasted of his murder plans on forums. Lanza, who had Asperger’s syndrome, also studied gun photos and obliterated virtual victims in violent video games such Call Of Duty for hours alone in his windowless basement den at the family’s £860,000 Connecticut home — to his mother’s despair.

Friends said Mrs Lanza, who had multiple sclerosis, often took mini-vacations on her own to escape the pressure of looking after her socially awkward younger son.

After a trip to Bretton Woods in New Hampshire, she returned to Newtown on Thursday night, just hours before her son cracked. Friends said the pair had not spoken for the entire time she was away because he was furious with her for trying to encourage him to leave the house, go to college or get a job and interact with the world.

He shot her in the face before driving to the school in a bullet-proof vest and black army fatigues, armed with her assault rifle, two handguns and hundreds of rounds of ammunition to carry out his massacre. He killed himself as police approached the school. A friend said: “He didn’t understand why she wanted him to go out into the world. She told me she couldn’t reach him and was worried. He was like a ghost.”

Police also believe Lanza may have gone berserk because his mother was planning to commit him to a mental hospital as she was increasingly unable to cope with his behaviour.