Now, researchers are offering a unique perspective.
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A national gun control group, Everytown for Gun Safety, indicates there is a common thread that links many mass shooters -- a history of domestic violence.
Everytown analyzed FBI data and media reports between January 2009 and December 2017 and found that 54 percent of those behind mass shootings killers had a history of domestic abuse or family violence. Twenty-six-year-old Devin Kelley, who killed 26 people at the church outside San Antonio, had been court-martialed by the Air Force in 2012 for assaulting his wife and fracturing his step son's skull.
The study also pointed to Pulse nightclub shooter, Omar Mateen, whose former wife told investigators he regularly beat her.
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James Hodgkinson, who shot congressman Steve Scalise and four others earlier this year, had a previous arrest for domestic battery.
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"There is no question we need to do a better job of keeping guns out of the hands of domestic violence abusers," says Jen Reidy, a Marin County mother with Moms Demand Action For Gun Sense In America.
Her group is an affiliate of Everytown, which conducted the research.
Beverly Upton, with the San Francisco Domestic Violence Consortium, believes reports of domestic violence should raise red flags.
"All restraining order loopholes should be closed," she says. "If you're the subject of a restraining order, you should not be able to buy or carry a gun."
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