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.

They also don't have military training. Veterans are justifiably angered by the Hollywood-driven meme of the unhinged vet who takes out his battlefield stress on his fellow Americans. (Thanks, Rambo.) In only four of the 29 cases did the shooter have any affiliation with the U.S. military, either active or prior at the time of the slaying, and the fusion center doesn't mention any wartime experience of the killers. Yet the Army still feels the need to email reporters after each shooting to explain that the killer never served.

It's harder to construct patterns around shooter motives, the report notes, since in most cases the killer takes his own life or gets killed by law enforcement before publicizing his reasons for violence. But DHS warns that "indicators of potential violence" include a worker's abrupt and persistent absenteeism; "escalation of domestic problems into the workplace; talk of severe financial problems"; a notable decline in "attention to appearance and hygiene"; unsolicited empathy with the perpetrators of mass violence; and vocalized musings about suicide. The fusion center doesn't offer more granular data.

The data comes as the country begins a renewed, heated debate about the relationship between mass-casualty events and easy access to guns. The Senate Judiciary Committee will explore the question in a Wednesday morning hearing that follows on a series of gun-control proposals pushed by President Obama in the wake of last month's Newtown, Connecticut elementary school massacre. It's worth noting that the fusion center study doesn't mention the circumstances under which the shooter obtained his guns.

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. That's despite an inability for federal law enforcement to track stockpiled firearms and literally years of al-Qaida sympathizers and propagandists urging disaffected U.S. Muslims to rise up against their neighbors.