This week, federal authorities have been investigating a series of bombs sent through the mail to high-profile critics of President Trump, including Barack Obama and George Soros. Two more, sent to Sen. Cory Booker (D-NJ) and CNN, were discovered this morning.

No suspects have been officially named, but the development continues to dominate the headlines. The Verge spoke to John Horgan, a terrorism expert at Georgia State University and the author of The Psychology of Terrorism, about the goals of terrorism, the best way to move on, and why we shouldn’t keep emphasizing that the mail bombs are crude.

This interview has been lightly edited for clarity.

First things first: what exactly is terrorism, and how do we distinguish it from other forms of violence?

Terrorism is a strategy used to instill fear. It’s a form of psychological warfare that uses violence or the threat of violence to create some social, political, or religious change. Terrorist violence, as opposed to other types of violence, is distinguishable because it serves a bigger picture. While the short-term effect is to intimidate, disrupt, or potentially kill, the point of terrorism is that there’s a broader message, and it’s targeted to a wide audience.

So, in a sense, something needs a pair of eyes on it to be terrorism?

Right. I might feel terrorized by a dog roaming the neighborhood, but that doesn’t mean that thing is a terrorist.

Is there a hierarchy for the most effective versus least effective forms of terrorism? What do we know about what spreads the most fear?

There is a tendency for novelty to capture our attention. We live in a country and a time when even mass shootings have become routine. Set against that, bombs are dramatic. They instill fear in a way that the threat of gun violence just doesn’t. And while there seems to be a very specific target profile here, the point about bombs is that even when they work, they’re highly indiscriminating. What makes it a form of psychological warfare is that anyone at the wrong place at the wrong time — like a postal worker or a neighbor to whom this might accidentally be left — might be affected. It feeds into the theater.

We’ve already seen an extraordinary volume of devices sent out in a short period of time. That in itself is quite unusual. This isn’t designed to be a one-off. That’s what the terrorist’s true power is, the feeling that he could strike again.

Does it matter, in terms of terrorizing the public, that nobody has been hurt yet?

We need to be very careful. There’s this talk of “false flags” and “rudimentary design” that can be tantamount to taunting. In the 1940s and ‘50s, there was a very interesting serial bomber terrorizing, George Metesky, dubbed the “Mad Bomber.” He planted something like 30 bombs. and it took the police over 16 years to catch the guy.

The point is, his devices were initially very crude. They didn’t work, but he got better and better, eventually developing devices that injured a whole bunch of people. We need to be careful about overstating the apparent ineffectiveness, and I’m positive that there’s a lot more that the FBI is not revealing to us at this time as well.

Given that terrorism does need a pair of eyes to exist, what do you think about the ethics of constant media coverage? I know this is something journalists think about a lot.

I think it’s unrealistic to expect we wouldn’t cover it, but it’s important to avoid rampant speculation about motive. Like I said, some aspects of the motive are obvious and clearly apparent that we can infer, but there’s more to this than what we’re currently aware of, and the reality is that we just don’t know a lot about this case.

There’s a lot being said about who’s to blame, and in my role as a psychologist, I would say, generally speaking, there’s a fine line between ideology and delusion. One of the common trends when you look at serial bombers is that they feel persecuted, they feel that there are malevolent forces plotting to against them, and there’s an urgency to respond. We live in such bizarre times, right? I think we cannot escape the fact that the president seems to be tacitly encouraging future disruptive acts.

That said, one positive thing that has emerged from studies of these kinds of actors is that, in most cases, other people know something about the person’s grievances prior to the event. In one study I worked on, we looked at 119 cases of the so-called “lone actor.” In over 60 percent of cases, friends, family, and co-workers were aware of the person’s intent because they had actually told them. In half of those cases, offenders were considered to be generally angry people. That certainly doesn’t narrow it down in this day and age, but I’m seeing a lot of the usual tropes about “isolated loners,” and that’s actually rare.

What exactly do these forms of terrorism do to us?

Terrorists might not be very effective at achieving their long-term goals, but the reason terrorism is such an attractive strategy for a wide variety of different actors is that, in the short-term, it can be remarkably effective. This is about making us pause.

If the goal is to spread the idea that anybody at the wrong place at the wrong time can be a victim, that accentuates the sense of vulnerability, so even though the intended target may be high-profile critics of the president. Terrorism is fundamentally about disruption, getting you to think “could I be next?,” getting you to pause before opening the mail, getting you to pause before getting on the subway, whatever it is. That’s the desired effect.

Earlier, you mentioned that gun violence almost doesn’t work as terrorism anymore because we’re so used to it. Does it take more to shock us now than it used to? And for people who are worried, what can they do to reassure themselves?

One of the paradoxes of terrorism as a strategy is that it’s very difficult to effectively maintain fear over a sustained period of time because the human body isn’t hardwired to cope with that. We return to some sort of baseline of functioning. One of the ironies here is that the audience tends to adapt pretty quickly and would just get on with our lives.

We still have to get the mail, we still have to go to work in the morning, we still have to go on. All of us have an in-built capacity to harness that resilience. So there’s a fine line to be walked between reassurance and vigilance. But it goes back to the age-old advice: just get on with it. Go on with your life. We are still far more likely to be hurt by the normal kinds of things that we’re exposed to every single day, like getting into a vehicle, and we shouldn’t be afraid of this.