In 1995, two men bombed a building in Oklahoma City, killing 168 people. It was the largest terrorist attack on American soil at the time.

A few days later, pollsters asked a simple question: How worried are you that you or someone in your family will become a victim of terrorism?

More than 40 percent said they were very worried or somewhat worried.

But over the next few years, without another mass bombing, that number steadily dropped.

Then 9/11 happened. The number spiked to 58 percent.

Much like with the Oklahoma City bombing, people calmed down in the following days, and the numbers began to drop. After a month, it was at 40 percent. It looked like fear would continue to dissipate until it got to pre-9/11 levels.

That's not what happened, though.

Instead, our fear levels stayed elevated

People's fear never got as low as it was before 9/11. In other words, we're just as afraid of being a victim of terrorism as we were one month after 9/11.

"There is almost no waning of fear, even before the rise of ISIS," says Ohio State political scientist John Mueller, who pointed out these trends in his book Chasing Ghosts: The Policing of Terrorism.

It's been 16 years since the attacks. There hasn't been an event remotely close to the scale of 9/11, and if you look at the number of terrorism fatalities in the US, they are a minuscule threat compared with things like car accidents, which kill 34,000 people a year. All evidence points to 9/11 being an "aberration, and not a harbinger," as Mueller puts it.

But year after year, when Americans are asked about how worried they are about terrorism, this evidence doesn't matter. And even if most Americans don't think it'll happen to them, most do think there will be a mass terrorist attack in the near future.

This dissonance between the threat and the fear is incredibly important. President Donald Trump signed an executive order suspending travel and immigration from Muslim-majority countries and barring nearly all refugees from entering the United States — and he did it in the name of national security. Opponents have pointed out that immigrant terrorism is a minuscule threat in this country, but there's a reason that fact doesn't matter to those who support the ban: they are still scared.

This is where it gets really hard: If there were an actual threat, we could remove it. But when the threat is in our heads — and when it's partially driven by residual fear from 15 years ago — alleviating the fear becomes much harder.

Why it's hard to extinguish fear of terrorists and witches

There are a few historical analogies to this elevated fear, like the Red Scare and Japanese internment, and they are all instructive.

But the most interesting analogy is Salem's witch hunt.

In 1692, three young girls in Salem Village, Massachusetts, began having violent fits. Townspeople believed the devil instructed witches to curse these girls. So they hunted down three women whom they suspected of being witches.

One of them confessed in hopes of a lighter punishment.

This affirmed a common Puritan narrative: that the devil wanted to infiltrate their community to destroy Christianity — and that witches were the devil's instruments.

In other words, this fear already existed.

This meant the devil had arrived and had to be exterminated.

This happened at a time when Salem's economy was struggling. King William's War in upstate New York and Canada forced an influx of refugees into Salem, causing economic pressure and social unrest. So Puritan villagers saw this as a sign of the devil's progress in destroying Christianity.

Meanwhile, the idea of witches existed in England long before colonists had settled in North America, so the character of witches was already baked into this narrative.

After one person confessed, villagers began wondering who else could be a witch. They accused elderly women, devoted churchgoers, and even a 4-year-old girl. Once the villagers tracked down potential witches, they put them on trial, and eventually executed 19 people.

But the problem is that killing some witches doesn't necessarily get rid of witches

Instead, it just makes you think there could be more of them. Anyone could be taken by the devil's influence, just like anyone could be a communist or a terrorist, and there's no way to know if the enemy is fully gone.

"What you're getting is a connection to a spooky foreign agency of sorts that you really can't tie down," Mueller says. "It's about ideology. You can't extinguish it, just like you can't extinguish witches."

So even after US troops killed Osama bin Laden, Americans returned to believing that the government wasn't doing enough to prevent further terror attacks. When the number of terrorism-related fatalities dropped to microscopic levels, we still didn't believe the threat was gone — especially since there continued to be smaller attacks.

And when it's unclear how to pin down an enemy, it's easy to overreach.

In 1944, after Japanese forces bombed Pearl Harbor, a poll showed that 13 percent of Americans believed we should exterminate all Japanese people, as if that would somehow make the country safer. Instead, the US incarcerated 120,000 Japanese Americans, many of whom had never been to Japan.

In short, ideas like a travel ban targeting Muslims have always resonated with some Americans in times of fear.

It's not just about the fear, though. It's about tying the fear into a story.

The Oklahoma City bombing elicited a spike in fear, but there was no greater narrative that developed and allowed us to frame it as part of something bigger. After 9/11, though, an implicit narrative developed out of the reaction.

While politicians, advertisers, and journalists didn't try to create this underlying fear of terrorism, Mueller says, "They find out what people are afraid of and take advantage."

That process results in cultural touchstones and national moments that weave together a bigger story. Some examples:

After 9/11, the consumer version of the Hummer, the H2, became available. It was a vehicle that was marketed as a "don't mess with me" car, implying that an external threat existed. In the book The Responsive Chord , advertiser Tony Schwartz talks about how these implicit cues allow audiences to complete the message, which makes it far more effective.

, advertiser Tony Schwartz talks about how these implicit cues allow audiences to complete the message, which makes it far more effective. On news broadcasts, virtually every terror-related event is covered as if it were part of a much larger narrative of Islamic terrorists slowly destroying America — and these broadcasts almost always have spectacular ratings.

The federal government tells us that we are still in a state of emergency, 15 years after 9/11.

During the election, both Trump and Hillary Clinton suggested that their opponent's national security strategies could lead to the downfall of America, which implicitly suggests that terrorists are at the brink of destroying the country.

Meanwhile, there have been smaller terrorist attacks and videos of ISIS beheading Americans.

And now, Trump is taking action to defend against this threat — implying that this threat exists on the scale of his reaction.

"The events can create raw fear, but the storytelling makes it refined fear," says NYU's Clay Shirky, who wrote Here Comes Everybody, a book about how groups organize themselves. (Full disclosure: I took a grad school course with Shirky about how to design spaces for group conversations.)

President Obama has tried to point out that our reaction to ISIS is actually harmful, because it's not as big of a threat as we make it out to be. But that didn't change the way Americans felt. So in the presidential campaign, Hillary Clinton tried to steer the conversation away from terrorism.

Because otherwise, what could she say? That our fear is unjustified? That even though 9/11 is the most resonant moment in American history, it was a fluke? That we should stop being afraid of things that scare us?

"You don't want to seem to be contemptuous of something that really frightens the American people," Mueller says.

Thinking about the narrative strengthens the narrative

Once you have the story, it gets repeated again and again.

It's the propaganda strategy Adolf Hitler coined in Mein Kampf, where he talks about concocting a lie so big that no one would believe it could be made up.

"'These people are our enemies' — repeat it over and over again — that they're not real Germans … and many people will believe it," says David Altheide, an Arizona State University sociologist who studies fear in media and politics.

And now, the place this narrative is repeated is online, specifically social media.

NYU's Shirky, who studies network effects, says that just a few years ago the news media would publish a story and the social media conversation would take time to catch up. But now more people use Facebook and Twitter to share reactions, as opposed to just the news story.

"When someone shares a link to Twitter, they are posting their reaction to it. Their reaction, not the headline, is where it gets metabolized," Shirky says.

The reaction to an event can frame it as part of the greater narrative in which America's way of life is under attack. No matter the event, if it can be fit into the narrative, that in itself becomes the news — not the actual singular event. In that kind of reframing, if a Muslim person commits a crime, it's not just that crime; it's an attack on America.

Altheide says this creates a narrative that is almost like a religion. It is unquestionable, it can't be falsified, and it's reaffirming.

In other words, no matter what happens, there becomes a way, always, to explain how the witches were involved. That's where we are now: We're trying to fight a threat that doesn't exist, but a fear that does.