Andrew Engeldinger took his motives to the grave Thursday, turning a gun on himself after shooting eight people at Accent Signage Systems in Minneapolis.
The same flash of workplace violence erupts 50 to 70 times every year across the country -- often with recurring patterns that shed light on the gunmen and what drives them.
The typical killer is a middle-age white man. He is socially isolated and feels justified in revenge when he gets laid off or passed over for promotion. And, most importantly, he leaves a trail of "bread crumbs" that preceded and could have even predicted his deadly rampage.
The hints include problems at home, depression and growing complaints -- or even threats -- about managers.
"It is rare that these things happen just because somebody was let go," said Mario Scalora, an expert in workplace violence and threat assessment at the University of Nebraska.
"Often, there is a trail of behavior that leads up to certain trigger points, and then you have the violence. There's usually some clues and bread crumbs."
The old notion of fired workers "going postal" and randomly killing co-workers in a rage is incorrect, Scalora and other scholars said. The key to preventing future tragedies, they said, is understanding the problems that precede violence and creating workplaces that can identify struggling workers and support them.
Some 500 to 600 workplace murders occur in the United States each year, most involving robbers or other assailants. But the second-most common circumstance is workers killing co-workers, customers or clients, according to a report released this month by the U.S. Bureau of Labor Statistics.