(NewsNation) — Monday’s attack by a Kentucky bank employee is the latest deadly shooting at a workplace, marking a continuation of a disturbing uptick in recent years.

Workplaces are the most common site where shooters carry out public violence, according to data from the Violence Project, and make up almost one-third of what the research organization defines as “mass shooting” locations since 1966.

Note: The Violence Project defines mass shootings as a firearm incident in which four or more victims are murdered — not including the shooter — in a single, public event not connected to an underlying crime like a robbery.

James Densley, co-founder of the Violence Project, said workplace shooters — who are defined as current or former employees — tend to share a few things in common.

“Workplace shooters are usually disgruntled employees,” said Densley, who also works as a professor of criminal justice at Metro State University. “They may have been fired from the workplace or are about to be fired from the workplace.”

In almost 90% of the workplace shootings analyzed by The Violence Project, the perpetrator knew some of the victims, and about 70% of the time employment issues motivated the attack.

On Monday, police in Louisville, Kentucky, say 25-year-old Connor Sturgeon, an employee at Old National Bank, opened fire inside, targeting people he knew with a rifle that was bought legally a week earlier. He killed five people and injured nine others.

The tragedy in Louisville marks a continuation of a disturbing rise in workplace shootings where four or more victims are killed. After surging throughout the mid-1990s and early 2000s, workplace attacks became less common throughout the 2010s, but that has changed recently.

“From 2020 to the present day, we’ve had about eight mass shootings in workplaces, and that would seem to indicate there’s an uptick in this type of violence,” said Densley.

In May 2021, a 57-year-old gunman shot and killed eight fellow Valley Transportation Authority employees at a light rail facility in San Jose, California.

Last November, a 31-year-old Walmart employee in Chesapeake, Virginia, shot and killed six people before turning the gun on himself.

According to The Violence Project’s research, shooters have killed four or more people in a public space 190 times from 1966 to Nov. 2022, and 58 of those occurred in the workplace. About 40% of workplace attacks — 22 in total — took place between 1993 and 2003.

Workplace attacks can present unique challenges from a security standpoint because shooters are more likely to be familiar with the building’s layout and its vulnerabilities. In many cases, workplace shooters also have credentials that provide them with access to restricted areas.

For that reason, adding more physical security may not be the best solution. Instead, Densley says, companies may look to HR procedures that would allow co-workers to report warning signs and intervene before an employee turns violent.

Police are still investigating what ultimately drove Sturgeon to open fire on his colleagues. The 25-year-old was taken down by police officer Cory Galloway within nine minutes of the 911 call.

A separate officer, 26-year-old Nickolas Wilt, was shot in the head and remains in critical condition. Wilt had only been on the job for 10 days before responding to the active shooter call.

The five victims killed Monday are Joshua Barrick, 40; Thomas Elliot, 63; Juliana Farmer, 45; James Tutt, 64; and Deana Eckert, 57.