First, the mental-health issue. A lengthy study by Mother Jones magazine found that at least 38 of the 61 mass shooters in the past three decades “displayed signs of mental health problems prior to the killings.” New York Times columnist David Brooks and Cornell Law School professor William Jacobsen have both suggested that the ACLU-inspired laws that make it so difficult to intervene and identify potentially dangerous people should be loosened. “Will we address mental-health and educational-privacy laws, which instill fear of legal liability for reporting potentially violent mentally ill people to law enforcement?” asks Professor Jacobsen. “I doubt it.”

Gun-free zones have been the most popular response to previous mass killings. But many law-enforcement officials say they are actually counterproductive. “Guns are already banned in schools. That is why the shootings happen in schools. A school is a ‘helpless-victim zone,’” says Richard Mack, a former Arizona sheriff. “Preventing any adult at a school from having access to a firearm eliminates any chance the killer can be stopped in time to prevent a rampage,” Jim Kouri, the public-information officer of the National Association of Chiefs of Police, told me last year at the time of the Aurora, Colo., Batman-movie shooting. Indeed, there have been many instances — from the high-school shooting by Luke Woodham in Mississippi, to the New Life Church shooting in Colorado Springs, Colo. — where a killer has been stopped after someone got a gun from a parked car or elsewhere and confronted the shooter.

Economists John Lott and William Landes conducted a groundbreaking study in 1999, and found that a common theme of mass shootings is that they occur in places where guns are banned and killers know everyone will be unarmed, such as shopping malls and schools.