Complicated and very nuanced — that's how researchers describe the challenge of tracking mass shootings and the traits of mass shooters throughout American history. This kind of study is in the spotlight once again, after another mass shooting in the United States.  

Police say Robert Card killed 18 people in Lewiston, Maine, on Oct. 25. The 40-year-old, who had served in the Army Reserves, was found dead of a self-inflicted gunshot wound two days later, according to the head of the Maine Department of Public Safety.

CBS News analyzed a database compiled by The Violence Project. The nonprofit tracks mass shootings, which it defines as four or more people murdered in one event, not including the offender. It shows out of 195 mass shootings since 1966, 50 involved suspects who were veterans or people with military training. 

To be clear, only a tiny fraction of people with military backgrounds become mass shooters. But military experience is something a disproportionate share of attackers have in common.  

That kind of nuance is what The Violence Project examines.  

"People with a military background are overrepresented as mass shooters in our data," said James Densley, co-founder of The Violence Project. "It bears saying that the vast majority of people who serve in the military go on to lead incredibly successful lives, and obviously, we're incredibly grateful for that service. And so, this is not a case that joining the military turns you into a mass shooter." 

Densley said the breakdown is important to consider as researchers continue to search for clues about how and why these mass shootings happen. 

A CBS News analysis of the data shows 26% of mass shooters over six decades have had military service or training. That is high compared to the general U.S. adult population, where 7%, or fewer than 1 in 10, has a military background, according to the U.S. Census Bureau. 

"Not all of them were deployed, and some of them only went through basic training, but they do have that throughline (military connection) in their histories," Densley said. "We actually have 14 or 15 mass shooters in our database that were marksmen or snipers in the military. I think there's some skills that are learned in the military, which may lend themselves then to mass shootings that follow. So, for instance, proficiency with firearms."  

Studies reveal there is never just one cause or reason for someone to commit one of these acts of violence. Often, there are mental health concerns as well, according to the analysis. 

"Many of these individuals actually experience behavioral challenges and, in some cases, mental health problems while they were still actively in the military," Densely said. "In some cases, that was actually the reason why they were discharged from the military and it seems to be that integration back into civilian life where these individuals have struggled piecing their lives back together again. And, I think it's a mix of all of that, which is probably what's underpinning some of these attacks." 

CBS News shared the data analysis with clinical psychologist Dr. Barbara Van Dahlen, who has worked extensively with the U.S. Department of Defense and military veterans for decades. 

Van Dahlen said it is not surprising people who have firearms expertise are overrepresented in this population. 

"It's complex," she said. "This is a very complicated set of factors, conditions, issues." 

In 2013, Van Dahlen appeared before the Senate Veterans Affairs Committee to testify on the state of veterans' mental health care across the country. She is now CEO of a tech company called WeBe Life, Inc., which helps people track their own mental well-being and get resources. 

"This is a very complicated set of factors, conditions, issues," Van Dahlen said. "When it comes to mass shootings, you know, there are certain people in our society who have more knowledge, more expertise. I might be somebody who wants to do great damage, but I might not have access to that kind of firearm and knowledge on how to use it, how to plan this as opposed to someone with a military background." 

What leads someone to do this? 

Research from The Violence Project identified other common traits of mass shooters, which include childhood trauma combined later in life with a personal crisis, which leads to radicalization. Access to guns is also common trait, The Violence Project found. 

Psychiatrist Dr. Ragy Girgis is an associate professor at Columbia University's Irving Medical Center in New York City who has focused some of his research on understanding mass shootings and the relationship between mental illness and mass shootings.  

"Just because a person who perpetrates a mass shooting has a mental illness doesn't mean that the mental illness is actually the cause of the mass shooting," Girgis said. "Once someone conducts a kind of motivational assessment of mass shootings" — as his team has done — "one realizes that in almost every case, the mental illness is incidental." 

Other research has shown that a history of domestic violence is also common among those who commit these kinds of attacks. 

In the aftermath of yet another mass shooting in America, experts hope this kind of research could help prevent future violence. 

"The more information we can have earlier, the more information we can have to understand where people, how people get to the point where they start to feel these kinds of impulses, their hopelessness, just giving up — and what can help change that course, so that it doesn't lead to something like this," Van Dahlen said. 

In:
• United States Military
• Mass Shootings
• Mass Shooting
• Maine