AMNA NAWAZ: Today marks 28 years since the bombing of the Alfred P. Murrah Federal Building in Oklahoma City, which remains the largest act of homegrown terrorism in U.S. history.

Judy Woodruff reports on how that act of political violence is being remembered and how the shockwaves of extremism are still felt today.

It's part of her series America at a Crossroads about the country's deep divisions.

JUDY WOODRUFF: This morning, a solemn scene, as family and friends of victims and survivors alongside citizens and public officials gathered at First Church in Oklahoma City to once again remember the lives lost, broken and forever changed at 9:02 a.m. on April, 1995, when an enormous truck bomb exploded next door in front of what was then a federal government office building.

As in years past, they observed 168 seconds of silence for each of the victims who died that day.

MAN: My father, Antonio "Tony" C. Reyes.

JUDY WOODRUFF: And read out their names one by one.

WOMAN: My daughter Carrie Ann Lenz and grandson Michael James Lenz III.

STEVEN TAYLOR, Former Chief Justice of Oklahoma Supreme Court: There is a breach in our country today, a division that must be healed.

JUDY WOODRUFF: Former Chief Justice of the Oklahoma Supreme Court Steven Taylor, who presided over the trial of one of the attack's conspirators, called on Americans to repair the breach.

STEVEN TAYLOR: We have an obligation to be good citizens, an obligation to respect democracy, to respect our government, our elections, our law enforcement, and respect for the social contract that we share with other citizens.

Never forget that these 168 were the victims of hate and violence and domestic terrorism.

JUDY WOODRUFF: Visiting the Oklahoma City National Memorial Museum after the ceremony was Dennis Purifoy, who 28 years ago was just starting his work day at the Social Security Administration on the first floor of the Murrah Building when the bomb went off.

DENNIS PURIFOY, Oklahoma City Bombing Survivor: To me, everything seemed to happen all at once.

I was knocked out of my chair.

Stuff fell on my head from the ceiling, and everything went black.

In fact, at one point, I thought, am I blind?

Because I could not see anything.

JUDY WOODRUFF: So this is what it looked like before?

DENNIS PURIFOY: Yes.

JUDY WOODRUFF: Miraculously, he suffered only minor injuries, but lost 16 of his Social Security colleagues that day.

Why do you think you survived?

DENNIS PURIFOY: Two things.

I was -- the bomb was on the north side of the building, and I was sitting more towards the south side.

And there were rows and rows and rows of cubicles between me and the bomb.

And I was sitting down.

One of my co-workers that died was 15, 20 feet from me, but she was standing up.

And so she was hit with some debris, and she died.

JUDY WOODRUFF: In the bomb's immediate aftermath, early news reports suggested links to international terrorism.

But the investigation quickly led back home to three American army veterans, Terry Nichols, Michael Fortier, and Timothy McVeigh, who planned and carried out the attack.

Following the Persian Gulf War and rejected by the Special Forces, McVeigh became disaffected, latching on to a growing pro-gun anti-government ideology that gained traction in the 1990s in the aftermath of federal law enforcement actions at Ruby Ridge, Idaho, and at the 1993 siege in Waco, Texas, where more than 70 died in a fire, as federal agents moved on the heavily armed Branch Davidian religious cult.

Timothy McVeigh was at Waco that day, observing from a hill nearby, and, exactly two years later, would bomb the federal building in retaliation.

DENNIS PURIFOY: I never did understand how he could think it would be -- anybody could think it would be OK to attack people who are trying to serve the public, serve the American citizens, just doing a job to support their families and to help people, how killing innocent people like that, civilians, if you will, that -- how that would accomplish anything.

But... JUDY WOODRUFF: Were you aware that there was a body of belief in the country that had these strong anti-government beliefs?

DENNIS PURIFOY: No.

No.

I was naive and innocent, like most people were, I think, and a lot of people still are.

KARI WATKINS, President and CEO, Oklahoma City National Memorial and Museum: Well, I think none of us can believe it.

I mean, it was an hour or so before we realized it wasn't a gas explosion, and it was a terrorist attack.

JUDY WOODRUFF: Kari Watkins is the president and CEO of the Memorial Museum.

KARI WATKINS: I think they decided they could have this attack on this quiet city in Middle America.

And I think our challenge is more relevant today than it even was in 1996, when we started.

JUDY WOODRUFF: The three men were prosecuted, found guilty, and, in 2001, McVeigh was executed.

In the years immediately after the attack, federal law enforcement cracked down on American paramilitary extremist groups, but that focus didn't last, says Michael Jensen, a terrorism researcher at the University of Maryland.

MICHAEL JENSEN, University of Maryland: These series of arrests really dismantled the paramilitary movement, and it was on the run, it was on the ropes.

And you can see this in the arrest data from the time period.

But then 9/11 happened.

And all of the resources that were being dedicated to tackling the domestic extremist threat were diverted to threats emanating from abroad.

And this was a really critical moment in terms of the development of the contemporary anti-government movement in the United States, because it gave it the space it needed to reorganize and to regroup and I think, most importantly, to identify new leaders.

JUDY WOODRUFF: Jensen's new research shows a dramatic rise in this type of violence since the 1990s.

From 1990 to 1994, there was an annual average of two attempted or completed mass casualty plots in America intended to kill or injure four or more victims.

From 2017 to 2021, that grew to an average of 40 a year, with a high water mark in 2020.

Most were planned or carried out by people associated with right-wing and anti-government groups and movements, the same ideology that propelled McVeigh.

MICHAEL JENSEN: What we have witnessed over the past five or six years is a tremendous increase in the rate of domestic extremist criminal activity in the United States.

For example, in 2021 alone, there were over 1,200 individuals that were arrested for extremist-related crimes in the United States.

A large number of these individuals participated in January 6 attack on the U.S. Capitol Building.

JUDY WOODRUFF: Jensen's study of January 6 defendants shows that about a third had ties to extremist groups, groups like the Proud Boys, the Oath Keepers, and the Boogaloo and 3 Percenter movements, many of which were founded by veterans of more recent wars and played pivotal roles in the storming of the U.S. Capitol that day.

MICHAEL JENSEN: What's really striking about Oklahoma City is how little has changed in terms of those dynamics.

FRANK FIGLIUZZI, Former Assistant Fbi Director For Counterintelligence: We're talking about people who generally look like us, worship like us.

They're our neighbors.

They're our uncles.

They're our cousins.

And so, historically, there has been an inability to acknowledge us as a threat.

JUDY WOODRUFF: Frank Figliuzzi is a former FBI agent who served as assistant director of counterintelligence for the bureau.

FRANK FIGLIUZZI: I like to say that, with regard to January 6, that was not an intelligence failure.

It was a failure to act upon available intelligence.

Part of that were legal constrictions.

And part of it was, no one seemed to acknowledge that Americans could do this to themselves and to our democracy.

JUDY WOODRUFF: In the decades since Oklahoma City, he says that the ideology that inspired McVeigh has only spread, aided in large part by social media.

DONALD TRUMP, Former President of the United States: Go ahead.

Who would you like me to condemn?

CHRIS WALLACE, Moderator: White supremacists and racists.

JOE BIDEN, President of the United States: The Proud Boys.

The Proud Boys.

CHRIS WALLACE: White supremacists and white militias.

DONALD TRUMP: Proud Boys?

Stand back and stand by.

JUDY WOODRUFF: And the mainstreaming of extremist ideology in a deeply partisan environment.

FRANK FIGLIUZZI: I don't believe we are at a place where we can come to agreement on the scope and depth of the domestic terror problem in this country, because the players that have to come to agreement, namely, our elected legislators, have to come to some kind of middle ground, where they acknowledge the problem.

And it seems to me we still aren't there yet.

JUDY WOODRUFF: Are you saying it's politics?

FRANK FIGLIUZZI: Well, I'm saying it's worse than politics, because we have we have managed to politicize a national security problem.

JUDY WOODRUFF: Figliuzzi points out that the Biden administration has created a national strategy for countering domestic terrorism, but, he argues, Congress needs to act, including finally criminalizing domestic terrorism to give law enforcement more tools to interrupt plots before they're committed.

FRANK FIGLIUZZI: We still, all these years after Oklahoma City, do not have a crime against domestic terrorism.

We don't have a law on the federal books against domestic terrorism.

JUDY WOODRUFF: Just last month, with multiple indictments looming, former President Trump kicked off his campaign for 2024 in Waco, during the 30th anniversary of the deadly Branch Davidian standoff.

Trump didn't mention the siege, but opened the event with a version of the national anthem sung by the J6 Prison Choir, a group of men imprisoned for their role in the January 6 attack.

And the former president continued to put forward anti-government conspiracy theories.

DONALD TRUMP: The abuses of power that we're currently witnessing at all levels of government will go down as among the most shameful, corrupt and depraved chapters in all of American history.

It's happening right before your eyes.

DENNIS PURIFOY: Just doing it in Waco is a signal to some people on the far right extremists.

JUDY WOODRUFF: Retired civil servant Dennis Purifoy.

DENNIS PURIFOY: I think it's a tragic what happened in Waco, don't get me wrong.

But it's also evident to anybody that knows anything about Timothy McVeigh and his motivations, one of the reasons he did what he did in Oklahoma City was because of what happened in Waco.

And, so for Trump to not acknowledge that in any way or disavow it, it was disgusting to me.

Still is.

KARI WATKINS: For me, when I stand in this room and I see these faces, there's not a better reminder of that we all have a responsibility to serve one another and to listen and to keep talking.

JUDY WOODRUFF: Back at the Memorial Museum, Kari Watkins says this place and this day should serve as a reminder of the many lives lost and forever changed 28 years ago to senseless violence.

Did you have any idea when you started this whole thing back in the 1990s that, all these years later, the country would still be dealing with the kind of attitudes that overtook Timothy McVeigh and his companions?

KARI WATKINS: No, I think we, when we built this museum, we talked about, there will be a day that would come that we would teach a generation that didn't live through this story.

And it seemed that it came very fast.

Our educators have a huge job.

And this museum has an enormous mission to make sure that we teach the senselessness of violence in a way that every kid can understand it and is willing to consume it.

It's a reminder that, today, in the midst of a divided country, that, on this sacred ground, we have to work to find common ground.

JUDY WOODRUFF: And it's hard.

KARI WATKINS: It's very hard.

JUDY WOODRUFF: A challenge for us today and for the many days to come.

For the "PBS NewsHour."

I'm Judy Woodruff in Oklahoma City.