Mass killers are likely to prefer handguns, are overwhelmingly likely to be male, and are unlikely to have served in the military, according to a November study by a Department of Homeland Security fusion center. Photo: Flickr/Anuj Biyani

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.

More: Here Are the Patterns the Feds Found for U.S. Mass Killings