The NRA blames violent films and video games, lack of armed guards and a mentally unstable perpetrator for the Connecticut shootings.
Gun crimes are also often attributed to drug abuse, alcohol, and economic inequality.
But guns are facilitators. "Our gun policies are designed, with few exceptions, to make the widest possible array of firearms available to the widest possible array of people, to use under the widest possible array of circumstances," says Dr Garen Wintemute, a gun violence expert and consultant to the US Centres for Disease Control and Prevention [CDC].
Although American violence levels are comparable to other developed nations, Wintemute says guns often change the outcome in the US.
"It's our homicide rate, not our violence rate, that is through the roof," he said. "An argument that might provoke a fist fight in Auckland leads to a homicide in Oakland."
The NRA has always insisted that "guns don't kill people, people do". This is disingenuous; guns make killing far easier. Nonetheless, the NRA mantra does hide a kernel of truth, albeit one the lobby is likely unaware of.
For although the vast US gun arsenal poses a major problem, a persuasive article in Mother Jones magazine links US violence to an environmental cause that is largely ignored by lobbyists and policy makers.
The key factor is lead. Lead toxins can retard brain development, affecting behaviour, especially in males.
The Mother Jones piece, written by Kevin Drum, found even moderately high levels of lead exposure are linked to aggressiveness, impulsiveness, attention deficit disorder, and lower IQ.
"And, right there, you've practically defined the profile of a violent young offender."
Although not everyone exposed to lead succumbed to crime, says Drum, he suggests "plenty of kids on the margin", slow and disruptive, were likely susceptible.
The chief source of lead in the post-war era was not paint but tetraethyl lead [TEL] added to petrol. Emissions soared from the early 1940s until the early 1970s, then dropped as catalytic converters and the Environmental Protection Agency [EPA] cut TEL use. It was banned in 1995. New Zealand followed in 1996.
Intriguingly, this U-shaped graph parallels a curve for violent crime, which rose from the 1960s through the 1980s, then dropped in the early 1990s. Last month the Justice Department reported a small decline in the prison population.
If you factor in a 23-year lag, writes Drum, drawing on research by Rick Nevin, vehicle emissions "explain 90 per cent of the variation of violent crime in America. Toddlers who ingested high levels of lead in the '40s and '50s really were more likely to become violent criminals in the 60s, 70s and 80s".
When Nevin looked at the correlation between lead data and crime rates in other developed nations, including New Zealand, he found the same pattern: violent crime peaked about 20 years after TEL use climaxed. The only dissenter to this argument has been the Ethyl Corporation - which makes tetraethyl lead.
The research explodes many myths, such as violent crime is worse in big cities. Absent lead-based petrol, crime rates in large and small cities is about the same. It also cuts across theories that attribute crime rates to demographics, economic conditions or drug use.
Significantly, it sinks race-based crime profiling that targets blacks: studies show black children raised near heavy traffic had higher blood lead levels than white children in low traffic areas.
Ironically, because lead residues are left in soil and can be disturbed, gentrification of poor areas means white children may be poisoned.
The EPA says no lead is safe; a tiny amount can harm IQ. A child who ingests any lead-contaminated soil may exceed safe levels.
Tulane University researcher Howard Mielke has mapped soil lead levels throughout New Orleans. He found high lead areas matched police data on high crime areas.
"We have the data on children's exposure, the lead in the environment, and the clean soil resources available to make changes in our city," says Mielke.
The response from criminologists to the astounding news that lead levels are linked to the rise and fall of violence has been - silence. Mark Kleiman, a public policy professor at UCLA, suggests this is because biology has been linked historically to racial theorising, such as the notion of an inherited criminal class, and because most criminologists train in sociology, not biology.
Reformers are also quiet. The Brady Campaign to Prevent Gun Violence, involved in discussions with the task force, had no comment.
Drum calculates that a 20-year programme to replace lead-painted windows and remove lead from soil will cost US$20 billion ($23.8 billion) annually. But persuading reformers to act on the lead-violence connection is a big task. "You're asking legislators to spend a lot of money on something that will reduce crime 20 years from now when they won't be in office. That's a tough sell."
Leaden facts
$250b
Estimated annual saving in the US from investing $23 billion a year in removing lead from soil and paint.
$70b
The share from kids with higher IQs
$180b
The savings from an estimated 10 per cent fall in crime.
