Guns are one constant in the recurring massacres in America, whether they are in places of education like here in Sandy Hook or at Virginia Tech five years ago or much earlier in 1999 at the high school in Columbine near Denver, or in cinemas, places of worship or shopping centres.
But as we strain to fathom what drove Adam Lanza – formally identified as the gunman last night – to kill 27, including his mother and 20 children, we look back at those attacks.
Some details about Lanza, who also killed himself, are starting to emerge. It seems he was a skinny, withdrawn and anxious person with high intelligence. His brother, Ryan, reportedly told police he thought his brother had a personality disorder, or possibly Asperger's, a form of autism.
That he was troubled hardly seems in dispute, raising questions about mental health care, how it is spotted and what treatments are available. The shooter at Virginia Tech, Seung-Hui Cho, had mental health problems that led to questions as to why he had been allowed to remain on campus.
There are clues here, but they do not bring us close to comprehension. Looking back to Columbine, investigators asked if the shooters were alienated by modern suburban life. Visit the Lanza home and the same questions arise. Number 34 on Yogananda Street, the house is large and impressive. But it seems lonely too.
When investigators found his computer at the house, the hard drive had been smashed, presumably by Lanza. He is said to have been a fan of computer games featuring warfare and killing. Did he cross some line from the fiction of these games into some twisted, self-realised reality last Friday?
But finally, we are back to guns. Ms Lanza also had a significant gun collection. Why? Tragically, it was from there that her son took his weapons.