There has been a tremendous amount of research about what causes a terrorist to pick up a weapon or build a bomb and, without discussing his plans or seeking affirmation from compatriots, lash out with violence — what we call lone-wolf terrorism. But while this violence has frequently been assessed based on political and social motivations, few have looked at its psychological underpinnings. That is the specialty of Joe Navarro, a former FBI agent and one of the founders of the agency’s elite Behavioral Analysis Program. The author of many books, including Hunting Terrorists: A Look at the Psychology of Terror and Interviewing Terrorists: The Definitive How-to Guide From an Ex-FBI Special Agent, Navarro has spent a lifetime studying the criminal mind. He has interviewed hundreds of terrorists and offers some profound insights for understanding what drives someone like Frazier Glenn Miller, an aging anti-Semite who in April 2014 allegedly went on a shooting spree at two Kansas Jewish facilities, killing three people, after a lifetime of racist activism. Whether discussing jihadist or radical-right terrorism, Navarro argues that all terrorists find justification for their actions through a fairly standard process that begins with the formation of a grievance against the outside world. Ultimately, they see violence as the only answer for addressing their grievance. No matter the lone wolf’s politics, Navarro says the psychology involved is essentially the same.

In your study of terrorism and those who commit lone-wolf attacks, have you found universal factors that guide their actions?

The one thing we know is that the psychology has always been the same. By that, I mean you have individuals who are collecting wounds, they’re looking for social ills, or things that have gone wrong, and they are nourishing these things that they’re ideating, that they’re thinking about. The solution for them is violence. The psychology is always the same.

What do you mean, collecting wounds?

Every terrorist I’ve ever interviewed, these people collect wounds. They will go back in history and tell you about the Crusades. That’s a thousand years ago, and they’ll tell you about it. They’re collecting these injustices and they just hang on to them, and they feed them and nourish them.

Doesn’t everybody, to a degree, collect wounds?

You and I don’t collect wounds. We experience wounds, but we don’t feed them and nourish them.

What then compels a would-be terrorist to start collecting these wounds that ultimately cause him or her to act out violently?

Every human is different. And what triggers one individual may not trigger his brother living under the same circumstances. We see this over and over. It’s a personal sort of thing. Either they experience it, they witness it, they start ideating it, or it’s imbued in them by someone, by a mentor, or they’re seduced by an ideology. I’ve talked to Palestinians who said they just grew tired of the fact that their father was being humiliated and searched every day, and that Jews didn’t have to go through that. As for terrorists in the United States, look at Ted Kaczynski. He had an enlightened education. He came from a good background, and yet a lot what drove him was his paranoia and his fear of technology. If you read his manifesto, basically what he is saying is that electricity is bad and that we should use pen and pencil and no computers. Wow.

Is this a universal template to understanding terrorism?

To say that there is one giant template that applies to everybody, well, there isn’t. What we do know is that all these individuals, what they have in common is that once they begin to ideate this philosophy, whatever their passion is, whatever their hatred is, whatever their ideology is, they certainly all begin to communicate this to people around them. And when we go back and do the post-event analysis, we find that they were talking about this, they were telling people about this, and the people either ignored it, didn’t pay attention or didn’t think it would go any further.

What happens then, when a person collecting wounds decides to follow through with their plans?

What we do know is that from ideation, they go to the next step, which is to isolate themselves, and when they begin to isolate themselves, both psychologically and physically, that’s when they became very dangerous. Because now they’re not getting a balance of ideas and thoughts. Now, they’re fulfilling themselves and their beliefs, and so they’re repetitively thinking about the same thing. Or they’re hanging out with a group that feeds that.

If there is no template to understand how this happens, what does that mean for law enforcement? How do they find someone on the edge?

Obviously, it makes it very tough. Because isolating yourself and ideating hatred in and of itself is not an offense. There’s a lot of people out there who hate passionately. The thing that makes it tough for law enforcement is if family and friends don’t keep tabs on this, then there is no way to identify where this potential threat may come from other than from the usual, which is groups, associations that are anti-this, or anti-that. Like the KKK. The FBI had penetrated the KKK so much that at one point, there were only informants. So you would have to penetrate these groups and listen to the chatter in hate groups and so forth. But what if you pull a Ted Kaczynski, what if you decide you don’t want to communicate with anybody? That becomes very tough. Law enforcement can never catch that guy.

What is that X factor that pushes an extremist to act on the wounds he has collected?

That’s a profound question. Under ideal situations, you’re tracking and you’re tracking. When did this individual start ideating this stuff, when did he start communicating this stuff, who are his associates, who is the biggest influence on him? Are they posting online? Are they increasing their posts? Is the verbal attack becoming more vicious? Does it look like they are psychologically crossing this line where they’ve convinced themselves that violence is the only thing to do? And keep in mind, when these individuals are in that froth, they have a lot of anxiety. And a lot of times they feel like the only way to relieve that anxiety is to kill somebody or to harm somebody.

As someone approaches that end, is there anything anyone can do? And is it possible to track how close someone is to that line?

You can’t keep track of everybody that is approaching the precipice and will cross over. So I don’t think that we have the answer. Theoretically, we have that model of how these individuals progress, but I don’t think anybody has a really good predictive model. Although this model comes close: Once they begin to isolate themselves, then they become increasingly more dangerous.