READ: The ritual of remembrance is different this year. Here's why.

A quarter-century after 9/11, the threat is coming from within.

I actually recall the sound of my shoes echoing off the floors of the U.S. Capitol. That musty marble has a sound (and smell) unlike any other. I was just a teenager from Indiana, selected to serve as a Congressional Page in the post-9/11 era. A messenger on Capitol Hill. Washington was a different place then. A heavy sense of duty hung in the air, and it transcended party lines. Members of Congress treated their work like wartime service. Gas masks were tucked beneath their chairs on the House floor, a grim reminder of the new threats facing our democracy.

That era was shaped by an external shock. An attack from foreign terrorists forced Americans to put aside partisan squabbles and confront a common enemy. In 2005, I stood at the back of the House chamber as President George W. Bush, fresh off a bitter reelection, addressed a room filled with Republicans and Democrats alike. The side that lost the election didn’t greet him with boos or jeers. They greeted him with handshakes. And he said something that still echoes in my mind.

"We have known times of sorrow and hours of uncertainty and days of victory," he declared, as I stood by the Page desk in the back. "In all this history, even when we have disagreed, we have seen threads of purpose that unite us."

Afterward, the applause was loud and bipartisan.

Caption: Every year I travel to Ground Zero at some point. That ritual of remembrance has new meaning in our current moment.

The unity we once felt seems like a relic. Twenty-four years after 9/11, the gravest threat to our democracy does not come from abroad. It comes from within. America worries not about its democracy being attacked by foreign hijackers, but by our fellow citizens. The violence is coming from inside the house.

There’s a statistic I use often because it illustrates what’s happened in our country: Threats against Members of Congress have surged tenfold in the past decade, from roughly 1,000 a year to nearly 10,000 last year, according to U.S. Capitol Police. These aren’t death threats to just Democrats. Or only Republicans. They’ve all experienced it, whether in voicemails and text messages or from menacing figures showing up outside their homes with pistols.

The list of plots has grown dizzying and more grotesque. We’ve seen assassinations of state legislators in places like Minnesota. We’ve seen plots to kidnap a sitting governor and hang a Vice President. We’ve seen weapons aimed at presidents and presidential candidates, at justices of the Supreme Court, at Members of Congress, and at media figures across the political spectrum.

Just yesterday, a prominent conservative commentator was gunned down in cold blood. As of this writing, the shooter remains at large, and his or her manifesto — if there is one — is yet unread. But we don’t need to see it. We know what motivated a heinous attack like this, and it was undoubtedly to send a message of intimidation through a despicable act of violence.

I spent years poring over these types of threats in the national security community. The ideological motivations behind these attacks shift and swirl, from jihadist radicalization to far-right extremism to far-left militancy. I’ve tracked American teens from the Midwest being groomed by ISIS, just as I’ve seen U.S. militia members plot to assassinate federal officials. And yes, we’ve seen fringe leftists take shots at the man who is now president.

You chase down the leads wherever they go and try your damnedest to attack the root causes of any violent motivation, which follow sick trends in our society like the undulations of a virus.

But the killers don’t come from one tribe. They come from all of them, and the consequence is the same. Censorship by shooter. When bullets fly to silence ideas, they aren’t more or less lethal because of what the shooter believed. What matters is that we all lose.

We are cowards if we don’t admit that we are all to blame. That’s not to say everyone condones political violence. The vast majority of Americans avowedly do not. But when it continues to happen — at this scale, with this ferocity — then it means we have failed miserably to thwart it. That’s on us.

To the ghouls who would glorify such atrocities, I say this: political violence is an attack on YOU as an American as much as it is on the target.

When a writer, a commentator, a lawmaker, or public servant is killed for speaking their mind, Americans are filled with fear and begin to self-censor. The town square empties out. History shows that the cycle of hate doesn’t stop; it see-saws. One side retaliates. Then the other. Until the blessings of liberty begin to boil away in the foul stew of vengeance. That is, unless leaders come together to calm tensions and strain for unity.

Years ago, I warned that this day would come.

“Political polarization will break records,” I wrote. “Look for stark declarations from pollsters and news anchors with phrases like ‘never before,’ ‘for the first time ever,’ and ‘unprecedented’ in describing how neighbors have come to see one another as bitter enemies.” I predicted that “technology will make it easier to dehumanize the other side, to crowd-cancel dissenters, and to accelerate the age of the deep-faked society, where lies become the reality.”

After that, I added, “real violence will arrive.”

“I see its feet under the door already…I’ve been hounded by stalkers, inundated with harassing messages, assigned bodyguards before speaking in front of security-screened audiences, and warned away from corners of my own country. My experience is becoming the norm for public servants and poll workers, but it’s nothing compared with what’s to come. Leaders will be killed, and the rifle shots will haunt us, while recriminations cause the republic to wobble.”

I wrote those words long before the recent wave of assassination attempts and killings swept our country. The republic is indeed beginning to wobble.

On this 9/11 anniversary, we owe it to the memory of the fallen — those lost in the towers, the Pentagon, the planes, and in the battles that followed — to confront the bitter truth. The gravest threat to free speech in this country isn’t coming from Islamic State terrorists or Russian hackers or Chinese spies. It’s coming from fellow Americans who are hellbent on silencing their neighbors, whether through official acts of political intimidation or despicable acts of political violence.

Those who died on 9/11 would bow their heads if they saw how we’re tearing ourselves apart. We promised we’d Never Forget them. But today I offer another exhortation. We must never forget who WE once were.

I came to Washington when our nation was united by principle, not ideology. Somehow we must find a way back. Even if we don’t do it for ourselves, we should follow the tradition of those who came before us and muster those thinning “threads of purpose” to do it for the next generation.