Many people work at dangerous heights, or with deadly chemicals or crushing equipment. But, as the gruesome killing of reporter Alison Parker and cameraman Adam Ward reminded us Wednesday, murder happens surprisingly often on the job. Out of nearly 4,600 workplace deaths in 2013, 9 percent were caused by homicides, according to the census of workplace deaths by the Bureau of Labor Statistics.

It's a pattern that disproportionately affects women. After car accidents, homicide is the most likely way for women to die at work, representing 21 percent of workplace deaths. Men, on the other hand, are more likely to die many other ways. Murders represent 8 percent of workplace deaths for men, preceded by car accidents, falls and contact with objects and equipment.

Overall, more men are murdered on the job than women. But that's because more men are killed on the job overall. Women do more than 40 percent of the work hours in this country, but represent 7 percent of workplace deaths, according to the census of workplace deaths. So more dangerous types of work result in 13 men dying on the job for every woman who dies.

In raw numbers, that was 4,261 men dying at work in 2013 to 321 women. There were 341 men and 67 women murdered on the job.

The murder threat for women is different. Both sexes die most often at the hands of robbers, and both also murdered at about the same rate by co-workers. But more than a third of women murdered at work are killed by boyfriends, spouses, exes or other relatives. For men, that category of killer is almost zero.

“When women are at work, their exes always know where to find them, don't they?” said security expert Chris E. McGoey in a telephone interview Wednesday.

McGoey said efforts to find a genuine motive for murders like Wednesday's are misguided. "You can't think logically. There's no sense to what they're doing. You can't connect the dots of what they're thinking and the decision tree because it's largely fantasy."

The share of workplace killings by co-workers appears to be inching up. The BLS changed its data collection between 2010 and 2011, so trends cannot be analyzed across that time. But the trend up to 2010 increased from below 10 percent of workplace murders to more than 12 percent. And under the new methodology, the share of killings by co-workers has gone up each year.

The Roanoke shooting came as a shock, but in retrospect the shooter appears to have shown at least seven of the the 13 warning signs of workplace violence, said Barry Nixon, executive director of the National Institute for the Prevention of Workplace violence.

While the killings are not frequent, it's the kind of threat that companies are more and more likely to be addressing, said Stephen Ward, who handles many forms of corporate risk management as East Coast operations manager for Pinkerton.

"Many companies may say, 'Hey, that can never happen here,'" he said. "People need to look at their workplace violence policies and work with their staff. It can happen any time."