Nobody knows why he did it. But that hasn't stopped them speculating. In retrospect, from the testimony of those who knew him, there were signs. But nobody could have predicted anything on this scale. What influences came to bear? What motives could there be? What would drive a young man to wilfully murder as many innocents as possible, leaving the country both vulnerable and mournful?

I pose these questions not of Dzhokhar Tsarnaev, the 19-year-old chief surviving suspect in the Boston bombing saga that left three dead and injured more than 170 last week, but of Adam Lanza, who shot 20 children and six teachers in Newtown, Connecticut in December. The contradictions in the political responses to the two tragedies and the issues they raise could not be more glaring or obscene.

On Monday the Boston Marathon was bombed. Within a day of suspects being identified, politicians who defended the status quo on guns were calling for "increased surveillance of Muslims" and addressing "loopholes in the immigration system" (Tsarnaev and his brother, Tamerlan, who was killed in a firefight, grew up in Kyrgyzstan).

On Wednesday the Senate declined to pass even the most anaemic gun control measures in response to the Newtown shootings. Twenty children, aged between six and seven, are slaughtered in school and the American polity takes five months to decide do nothing. Unable to break the filibuster limit, it didn't even come to a vote. Hiding behind the National Rifle Association's (NRA) talking points, gun rights senators cloaked themselves in the constitution, insisting support for gun control would violate the second amendment "right to bear arms".

While the authorities denied the still unconscious Dzhokhar his Miranda rights (informing him that he has the right to remain silent), some Republicans insisted he be tried as "an enemy combatant" – the legal aberration and moral abomination that paved the way to Guantánamo Bay. Their devotion to constitutional rights, it turns out, is partial; their embrace of guns is complete. The NRA even opposes legislation banning gun sales to people on the terrorist watchlist, meaning those who can't board a flight can still lock and load.

As John Oliver, a UK comedian on the Daily Show, noted: "One failed attempt at a shoe bomb and we all have to take off our shoes at the airport. Thirty one school shootings since Columbine and no change in the regulation of guns."

The ramifications of this neglect cannot be overstated. More than 85 people – including eight children – are killed with guns on an average day in America and more than twice that number are injured. Even taking into account the fact that most gun deaths are suicides, that's still several times the death toll of 9/11 every year.

Numbers alone, however, don't quite do the cognitive dissonance justice. The effect of a terrorist attack such as Boston cannot be measured in the number of slain alone. Terrorism creates a culture of fear and suspicion that spreads beyond those immediately affected and impacts upon our understanding of risk. It means no one feels safe, anyone is a potential suspect and danger could be anywhere. "Terror is first of all the terror of the next attack," writes Arjun Appadurai in Fear of Small Numbers. "Terror … opens the possibility that anyone may be a soldier in disguise, a sleeper among us, waiting to strike at the heart of our social slumber."

The trouble is this is precisely the culture that many Americans have lived in for years. It's estimated that in Chicago 20-30% of the children have witnessed a school shooting. Carolyn Murray, whose son was shot dead on his grandmother's lawn in a Chicago suburb, does not enjoy much in the way of "social slumber". She has become so accustomed to gunfire at the weekends she could call the police and tell them what calibre was used and the direction of the shooting, just by listening in bed. Put bluntly, a significant section of America lives in constant terror and Congress just decided they should continue to do so.

In the hierarchy of suffering and security in America there are, in short, places where you are supposed to be safe – marathons, suburban schools, cinemas – and places where you are apparently entitled to no such expectation: particularly poor black and Latino neighbourhoods. Only in a handful of exceptions – when the killers are white, American-born Christians (the media has developed no default anxiety about them), the dead mostly white and the murders in large numbers – do shootings stand a chance of attracting mainstream political attention. Even then, only rarely, and even then, as last week proved, to little tangible effect.

The uncomfortable reality is that there is precious little that can be done to prevent an atrocity such as that in Boston. "We've had a lot of successes in degrading the ability of al-Qaida to launch massive attacks," Democratic congressman Adam Schiff, a former federal prosecutor and member of the house intelligence committee, told the Washington Post. "But we've had a proliferation of one-off, foreign-born plots and self-radicalised individuals … We're going to have to recognise a certain vulnerability, and adopt a determined view that we will go on as we have, taking prudent precautions, but not changing the way we live."

Conversely there is a great deal that can be done to change the way Americans die daily. Gun rights advocates insist the gun-control measure that had the best chance last week, which demanded background checks for guns sold online and at gun shows, would not have prevented Lanza's crime. He used his mother's gun, which had been purchased legally. That's true, although since 90% of Americans and even 74% of gun-owning members of the NRA agreed with it, they should have passed it anyway. But the reason the compromise proposal was so tepid is because they scuppered efforts to ban assault weapons and high-capacity magazines, which enabled Lanza to kill far more children far more quickly.

Expressing frustration at the failure to pass anything, Andrew Goddard, whose son Colin was injured during the 2007 shootings at Virginia Tech, told the New York Times: "It's almost like you can see the finish line, but you just can't get there. It's more annoying to be able to see it and not get to it."

There's many a Boston marathon runner who knows exactly how that feels.

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