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.

While overall workplace deaths have declined over the past 20 years, the number involving workers killing co-workers hasn't changed.

In Minnesota, 95 people were murdered at their workplaces between 1992 and 2010. Of the victims, 68 were shot. Thursday's shooting was Minnesota's first in at least two decades in which a workplace homicide involved more than two victims.

Killers have a strategy

When disgruntled workers kill colleagues, they typically are seeking revenge and are strategic in their plans, their targets and intentions to then take their own lives, said James Fox, a criminologist at Northeastern University in Massachusetts.

Most of the killers live alone, are socially isolated and blame others for their problems, he added.

"It takes more than just one disappointment in life," Fox said. "There's a history of failure and frustration, and you tend to find someone who externalizes blame. Some of us blame ourselves when things go wrong. Others blame everyone else -- 'The boss doesn't give me good assignments' or 'The guy at the next desk is always taking credit for everything I do.'"

Experts interviewed last week stressed that they were talking generally and not about the Accent shooting.

In August, a laid-off employee shot his former manager outside the Empire State Building. Nine bystanders were hurt, and the shooter was killed by police. A subsequent investigation found a history of problems between the worker and the manager. In 2010, a beer distributor killed eight people at his worksite in Manchester, Conn., after he had been fired.

Some hallmarks of workplace shootings fit the Accent case. Engeldinger lived alone in Minneapolis and, one uncle said, had been estranged from family. Relatives also believed he was suffering from a mental illness and discussed getting him help.

Knowing the basic profile of workplace shooters still isn't very helpful in preventing workplace homicides, Fox said, because there are millions of workers who blame others or get laid off or suffer depression and don't turn violent.

"The warning signs are only yellow flags, not red flags," Fox said. "The yellow flags only become red flags after blood has been spilled."

Help for employers

The Minnesota Department of Labor and Industry has responded to concerns about workplace violence with training sessions, a hot line and a website for employers. The department has fielded more inquiries lately about security in health care facilities and in public places such as courthouses -- particularly after the courthouse shootings in Grand Marais, Minn., in December, spokesman James Honerman said.

An estimated 1,000 work-related assaults happen in Minnesota each year.

Larger companies can employ more security and human resources protections. But history suggests they are equally at risk for workplace homicides as small companies such as Accent, said Scalora, who served on an FBI work group on workplace violence.

While angry workers might not be able to get to the top executives they blame, "that doesn't mean other people might not be at risk," Scalora said. "Just because you can't get to the boss doesn't mean that other people might not get hurt."

Companies should develop a sensitive approach to laying off workers, said Carolyn Reinach Wolf, a New York attorney who specializes in mental health law.

"Organizations need to learn how to fire people ... and to think about the person behind the job title. It's not just the assistant director of marketing. There's a name behind it."

Wolf recommends employee assistance programs, which provide support and counseling to troubled workers, and risk assessment programs, which encourage people to report their concerns about co-workers.

"If you really look at [workplace homicides], there are warning signs," she said. "Organizations need to look at those, take them seriously and set up systems to intervene sooner rather than later. What's the downside if you intervene and nothing happens?"

Fox said those strategies are necessary but insufficient and potentially dangerous if used incorrectly. Identifying threats could alienate at-risk workers if employers don't know how to listen to them and treat their concerns with respect.

"If you really do try to identify the few people who are the so-called 'co-workers from hell,' you can actually precipitate the very violent act you are trying to prevent," Fox said. "Because then they are going to feel like they are being persecuted."

Assessing the risk

Agencies such as Minnesota Threat Assessment and Forensic Professionals can consult with companies to identify the risk levels of problem workers and create safe and appropriate responses. Agency founder and psychologist Kristine Kienlen said responses range from police visiting employees at home to counseling for workers to one-on-one sessions where workers air their grievances.

"For those people who are struggling because of losses, because of mental health issues, sometimes getting the support that they need is what is going to deter them from moving down the pathway of violence," she said. "Sometimes just giving them the opportunity to have their side of the story told will help."

The ultimate prevention strategy, Fox said, is to foster work environments in which employees are respected, bullying isn't tolerated and concerns can be safely expressed. Workers also need balance, he said, and can't be so tied to their offices that work becomes their lives.

"You don't ever want work to be everything," Fox said, "because when your work is everything, you know what can happen when you lose it."

Jeremy Olson • 612-673-7744