Spencer Ackerman at Wired sums up the conclusions of a U.S. government "fusion center" report on what we think we know about mass shooters from Columbine on. Highlights:

This is what a mass killer looks like, according to a Department of Homeland Security analysis. He works alone. He uses a semi-automatic handgun. He’s a he. And he probably didn’t serve in the U.S. military.

That’s the conclusion of a November 28 analysis by the New Jersey branch of the Department of Homeland Security’s partnership with state and local law enforcement. The so-called intelligence “Fusion Center” sifted through data on 29 major mass killings in the U.S. since 1999, starting with the Littleton, Colorado school shooting. Its practical advice is to be more concerned by your co-worker with the bad hygiene who mutters about putting his “things in order” than by the war veteran in the next cubicle.

The basic pattern found by the New Jersey DHS fusion center, and obtained by Public Intelligence(.PDF), is one of a killer who lashes out at his co-workers. Thirteen out of the 29 observed cases “occurred at the workplace and were conducted by either a former employee or relative of an employee,” the November report finds. His “weapon of choice” is a semiautomatic handgun, rather than the rifles that garnered so much attention after Newtown. The infamous Columbine school slaying of 1999 is the only case in which killers worked in teams: they’re almost always solo acts — and one-off affairs. In every single one of them, the killer was male, between the age of 17 and 49.

So, what can we learn in public policy terms from this knowledge about how to stop these events in the future? Pretty much nothing. Even as we muse from Senate to drawing room over how access to guns makes or might make such crimes more likely, Ackerman writes, "It’s worth noting that the fusion center study doesn’t mention the circumstances under which the shooter obtained his guns." While I don't know their official reason, a good guess is because in a world where guns and a Second Amendment exist, it doesn't really make any difference. And Ackerman's zinger conclusion:

One of the most striking patterns about U.S. mass killings is visible only through its absence. Terrorists aren’t committing these crimes. Ordinary, unhinged American men are.

Jesse Walker on mass shootings and on the general uselessness of fusion centers.