The gut-wrenching shock of the attack at Sandy Hook Elementary School on December 14 wasn't just due to the 20 unthinkably young victims. It was also due to the realization that this specific, painfully familiar nightmare was unfolding yet again.

As the scope of the massacre in Newtown became clear, some news accounts suggested that mass shootings in the United States have not increased, based on a broad definition of them. But in fact 2012 has been unprecedented for a particular kind of horror that's been on the rise in recent years, from Virginia Tech to Tucson to Aurora to Oak Creek to Newtown. There have been at least 62 such mass shootings in the last three decades, attacks in which the killer took the lives of four or more people (the FBI's baseline for mass murder) in a public place—a school, a workplace, a mall, a religious building. Seven of them have occurred this year alone.