Note: The original version of this post stated that there have been 43 mass shootings in 25 states since 2009. These numbers have been updated to reflect a revised version of the study referred to below.

The massacre of 12 people Monday morning at a navy yard in the nation's capital was exceedingly tragic but also alarmingly familiar.

“We are confronting yet another mass shooting,” President Obama said wearily in a briefing later that day.

A study published in January by the gun control advocacy group Mayors Against Illegal Guns* found that more than 50 mass shootings in 30 states have occurred since Obama took office in January 2009. A sizable uptick from previous years, that's a rate of more than one per month with an average of six fatalities per incident (in which a "mass shooting" is defined as an incident where four or more people are killed). And even since that report was published, several lesser-covered mass shootings have occurred n 2013.

According to the report:

In over half of the incidents, the shooter killed a current a former intimate partner or family member.
Assault weapons or high capacity magazines were used in less than a quarter of mass shootings, but when they were used, more than twice the number of people were shot and 57 percent more were killed.
Less than a quarter of the shootings occurred in public spaces that were so-called ‘gun-free zones.’
In only a very small numbers of these shootings were mental health concerns about the killers raised with authorities beforehand.

Explore more posts about this issue, including maps, charts, and an educator guide

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A horrific incident at an elementary school in Newtown, Conn. last December, in which 20 children and seven adults were slain, shocked the nation and sparked calls for stricter gun laws. But the fierce political debate that ensued, and the potential for new gun control regulation, virtually evaporated from headlines after the U.S. Senate failed to pass a bipartisan bill on background checks.

Perhaps the most distressing finding in the study is that victims of mass shootings make up less than 1 percent of all gun deaths in the United States: in 2010 alone, roughly 8,775 people were killed by a firearm, according to FBI data.

* Mayors Against Illegal Guns, founded by New York City Mayor Michael Bloomberg, advocates for stricter gun-control laws. In the report, the group lists every single mass shooting, along with related details about each incident.

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