Mass shootings in the workplace are rare and puzzling

A postal worker on disability retirement for psychological issues returned to her workplace years later and killed six people. A Connecticut beer delivery worker irate over being forced to resign opened fire as he was being escorted out of the building, killing eight co-workers. Just a week ago, a longtime municipal engineer submitted his resignation in the morning and within hours went on a rampage inside the building, killing 11 co-workers and a contractor.

The Virginia Beach killing is one of 11 mass workplace killings dating back to 2006 in the U.S., according to a database of mass killings maintained through a partnership between AP, USA Today and Northeastern University.

In all, nearly 90 people have died in these mass shootings, defined as 4 or more people killed, not including the perpetrator. And while such workplace shootings remain rare among the tens of thousands of gun deaths each year, they resonate among Americans who worry they might become an aggrieved co-worker's next victim.

The phenomenon steadily seeped into the public's consciousness following a series of worksite massacres, including a big one at a post office that eventually led to the term "going postal" being coined in the 1990s. The U.S. Postal Service went to great lengths to examine the issue and dispel the notion that its workers were more prone to violence.

Here are some of the key takeaways from the data:

WHAT ARE THE REASONS PEOPLE KILL CO-WORKERS?

Several factors have led up to these mass killings: they include disagreements on the job, feeling mistreated by bosses and colleagues and anger in the aftermath of court proceedings.

"Most of the time there are all sorts of signs of what the grievances were," said James Fox, a criminologist with Northeastern University who has studied mass...