When a premeditated massacre occurs – such as the terrifying carnage in Las Vegas or the heartbreaking murders in the Sandy Hook elementary school – journalists, politicians and millions of ordinary people turn their attention to the families that raised the killers, the siblings that shared their childhoods, and the wives or girlfriends who lived with them.

Particularly when the killers commit suicide, taking their dark secrets to the grave, those who were close to them are often the only window we have into their motivations. But in the absence of a living person to blame and a figure against whom we can vent about the pain and the loss, their nearest relations are also the people at whom we can point fingers.

Having spent two years studying rampage shooters in American middle and high schools, I saw the same pattern of “moral archaeology” unfold. The families of the shooters were looked upon as the proximate agents of the shooters’ acts, not because they pulled the actual trigger, but because commentators assumed they might have been pulling emotional triggers for years in advance.

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Did they pressure their sons? Mock them? Drive them into seclusion? Did they know their sons were killers-in-waiting and if they had any hints, why didn’t they act to protect the community that surrounds them?

Within minutes of learning about the shootings I studied, the families knew they would be confronted with these questions. Yet, they were almost as much in the dark about why these terrible events had happened as their neighbours or the police. These young shooters were masters of concealment, capable of planning their mayhem for months in advance without evoking suspicions.

In part this was because they were living in an interior world filled with demons that they well knew were signs of illness or deviance. They didn’t want anyone – least of all their closest loved ones – to know how quickly they were descending into psychological chaos.

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Being labelled a “sicko” is not what any teenager wants. And these teens were already having trouble fitting in. What they most wanted was peer acceptance and it was among peers that the clues were most visible, not necessarily about the murders to come, but about the driving desire for social acceptance. Sadly, the pathway to peer affection lay in plotting a spectacular event that would change the definition of their social persona from “loser” to “notorious”.

For older shooters, though, the dynamics of peer group acceptance are no longer in play. Hence they give off even fewer clues to the people around them. Not their families, not their friends.

They may display odd behaviour in the workplace – which was the gambling casino for Steven Paddock (or the post office for others). Workers would often say they were gruff, hostile, and prone to verbal outbursts. But millions of people fit this description and only a tiny handful express their hostilities with guns in their hands.

Families of older shooters are often concerned about them or wary of them because of these socially awkward behaviours, but they rarely imagine this conduct is a precursor to murder because 99 per cent of the time it isn’t.

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

Moreover, parents and girlfriends have a very limited “data base” for assessing the behaviour of their father, brother, or lover against a range of cases that might tell them how far outside the margins of normal the concerning behaviour might be. They just know that Adam Lanza doesn’t have any friends (Sandy Hook) or that Steven gives people a hard time if they cross him in the casino (Las Vegas). But these signals are faint and rarely lead to psychiatric treatment or a police file.

Still, because we don’t like to think the world is that hard to fathom, we often blame families in the aftermath. If they were truly good parents, they wouldn’t have raised a killer. Because the families know this moral terrain, they are forced into a difficult choice.

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Gretchen Woodard, the mother of Mitchell Johnson who killed his Arkansas classmates and a teacher in Westside Middle School, took public responsibility for her son even though she had no idea he was about to kill anyone. She attended church services after the shooting, apologised before her community, explained that she loved her son even though she condemned his actions, and after doing as much penance as she could, asked for their forgiveness when her small rural community was ready.

The family of Andrew Golden, Mitchell’s younger accomplice, took the opposite approach: they literally headed for the hills, secluded themselves, and attempted to remain permanently out of any spotlight.

Understanding the pathway to murder, especially on such a massive scale, is important because living with randomness is psychologically and politically impossible when the costs are so high.

If we know that a killer spent his formative years in the company of a pathological father who was a bank robber – as now appears to be the case for the Las Vegas perpetrator, Steven Paddock – we can imagine how such a warped childhood may have produced a sociopath who would kill 58 people and maim over 500 others. It doesn’t reduce his moral culpability, but it makes us feel that the social world is more predictable.

When we learn that he shielded his girlfriend by sending her away to the Philippines before setting up his arsenal and that she knew him as a “kind, caring, quiet man” – then we also recognise his capacity for cold-blooded planning and his expertise at duplicity.

His methodical approach tells us he was not an impulsive person. And we feel both terrified and comforted: terrified because who knows how many caring, quiet men there are in our midst who are also capable of mass murder and comforted because it is very hard to predict or stop someone who is this dedicated to an evil end. If his loving girlfriend didn’t have a clue, how could any of the rest of us?

Family members bear the suspicion and the blame, especially in the absence of the killer, and the experience haunts them for the rest of their days.