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BOISE, Idaho — Their anger is all over social media for the whole world to see, with rants about minorities, relationships gone bad or paranoid delusions about perceived slights.

The perpetrators of mass shootings often provide a treasure trove of insight into their violent tendencies, but the information is not always seen by law enforcement until after the violence is carried out. In addition, rants and hate speech rarely factor into whether someone passes a background check to buy guns.

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The massacre at a Pittsburgh synagogue, the pipe bombing attempts from last week and the Florida high school shooting this year have underscored the dilemma of law enforcement around the country in assessing the risk of people making online rants at a time when social media has become so ubiquitous.

“We can go out on Twitter and there are loads of people saying insane stuff, but how do you know which is the one person? It’s always easy after the fact, to go: ‘That was clear.’ But clearly everyone spouting their mouth doesn’t go and shoot up a synagogue,” said David Chipman, a retired agent of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and now senior policy adviser for the Giffords Center.