Eighty-two percent of weapons involved in mass shootings ... bought legally...
A vast majority of guns used in 16 recent mass shootings, including two guns believed to be used in the Orlando attack,
were bought legally and with a federal background check.
At least eight gunmen had criminal histories or documented mental health problems that did not prevent them from obtaining their weapons.
http://www.nytimes.com/interactive/2015/10/03/us/how-mass-shooters-got-their-guns.html?_r=0
Certain categories of people, including felons, certain domestic abusers, and severely mentally ill people are prohibited by federal law from possessing guns. There was sufficient evidence to judge whether the shooter was a prohibited gun possessor in 116 of the 133 incidents (87%).
Of those 116 incidents, 44 (38%) involved a prohibited possessor, and 73 (62%) did not.
https://everytownresearch.org/reports/mass-shootings-analysis/
Eighty-two percent of weapons involved in mass shootings over the last three decades have been bought legally,
according to a database compiled by Mother Jones magazine that defines a mass shooting as taking the lives of at least four people in a public place.
http://www.nbcnews.com/storyline/san-bernardino-shooting/more-80-percent-guns-used-mass-shootings-obtained-legally-n474441
The Washington Navy Yard shooting a year ago that left 12 victims dead was one of more than five dozen mass shootings and spree killings in the U.S. since 1984. The shooter, Aaron Alexis, who also died,
was armed with a legally obtained shotgun and a .45-caliber handgun taken from a security guard at the scene.
Alexis had told police earlier that he’d heard voices, and had received mental health treatment.
http://www.washingtonpost.com/wp-srv/special/national/weapons-and-mass-shootings/
A Guide to Mass Shootings in America
There have been at least 82 in the last three-plus decades—and most of the killers got their guns legally.
Of the 143 guns possessed by the killers,
more than three quarters were obtained legally.
The arsenal included dozens of assault weapons and semi-automatic handguns with high-capacity magazines.
(See charts below.) Just as a perpetrator used a .40-caliber Glock to slaughter students in Red Lake, Minnesota, in 2005, so too did the one in Aurora, along with an AR-15 assault rifle, when blasting away at his victims in a darkened movie theater. In Newtown, Connecticut, the attacker wielded a .223 Bushmaster semi-automatic assault rifle as he massacred 20 school children and six adults.
http://www.motherjones.com/politics/2012/07/mass-shootings-map
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