Mass murderer Adam Lanza brought three guns into Sandy Hook Elementary School in Newtown – a Glock-10 pistol, a Sig Sauer pistol and an AR-15 semi-automatic rifle. What distinguishes them is that they all have high-capacity magazines that hold 15 to 30 bullets and can be reloaded in seconds.
The high-capacity magazine, designed for use by police and the military, has become the hallmark of mass shootings.
James Holmes, who killed 12 people at a movie theatre in Aurora, Colo., last July, also used a AR-15 semi-automatic rifle with a 100-round capacity, as well as two .40-calibre semi-automatic pistols.
Jared Loughner, who killed six people and gravely wounded former congresswoman Gabrielle Giffords in a Safeway parking lot in Tucson, Ariz., in January, 2011, used a Glock-19 pistol with a 33-round magazine.
Eric Harris and Dylan Klebold, who killed 13 at Columbine High School in Littleton, Colo., in April, 1999, used a TEC-DC9 assault pistol and a 9mm Carbine piston, both with high-capacity magazines, as well as two pump-action shotguns.
Seung-Hui Cho, who killed 32 at Virginia Tech University in Blacksburg, Va., used a Glock-19 pistol and a Walther P22 pistol, both with 15-round magazines.
Assault weapons of this sort were actually banned in the U.S. from 1994 to 2004 but, as the data show, that didn't prevent the use of this type of weapon.
A proposal in March, 2011, in Connecticut would have made it a felony to possess magazines with more than 10 bullets and required owners to surrender them to law enforcement or remove them from the state. Opponents sent more than 30,000 e-mails and letters to state lawmakers as part of a campaign organized by the NRA and other gun advocates, said Robert Crook, head of the Hartford-based Coalition of Connecticut Sportsmen, which opposed the legislation.
"The legislators got swamped by NRA e-mails," said Betty Gallo, who lobbied on behalf of the legislation for Southport-based Connecticut Against Gun Violence. "They were scared of the NRA and the political backlash."
As the debate heats up around gun control – as it does after each of these incidents – there are mixed messages to be taken from the data about firearms and increasingly their lethal power.
While gun ownership has soared in the U.S. in recent years, violent crime, including the murder rate, has actually fallen pretty steadily.
However, this is not necessarily a cause and effect. Consider that, in the wake of 9/11, security measures have been tightened across the U.S. and the number of police has soared.
In the search for answers after such a horrific massacre, it has been noted too these mass killings are invariably carried out by young men. (Of 62 mass killings over 30 years catalogued by Mother Jones magazine, only one was perpetrated by a woman.)
Young men are among the hardest hit by the economic downturn and the biggest consumers of violent video games, which has also sparked much speculation, though there is little solid research on possible links.
The vast majority of shooters have also suffered from untreated mental illness, which has become another trait of these mass murders, and a point of fierce debate.
More than anything, however, the parsing of the data about violence, and gun violence in particular, reveals that there are no easy answers, no clear way of determining who the next Adam Lanza could be, no easy way of preventing another tragedy.