The would-be assassin’s gunfire that wounded former President Trump and sent panic through a Pennsylvania campaign rally on Saturday was unleashed at a time of increasing hostilities in American politics that have roused extremes on the left and the right and further inflamed divisions ahead of the November election.

Multiple shots rang out across a field. A bystander was killed and two were seriously wounded. The FBI identified the shooter as Thomas Matthew Crooks, 20, who was killed by a Secret Service sniper, and in the hour before sunset, a narrative of blame, recrimination and suspicion was amplified on social media and partisan TV talk shows.

Law enforcement agents are investigating Crooks’ motives, but as of Sunday evening officials say it appears he acted alone. Whatever his intentions, they come when the threat of political violence is more acute and disparate than it has been in a generation. In the decade immediately after Sept. 11, 2001, terrorist attacks, the nation’s focus was on networks of international Islamic militants. That danger has largely been eclipsed by a troubling spectrum of homegrown extremists of all persuasions looking to strike out against a country they see as failing and corrupt.

“It becomes a numbers game. You only need one person to do damage,” Colin P. Clarke, the director of research at the Soufan Group, an intelligence and security consulting firm in New York City, said in a recent interview. “I see parallels with the left-wing groups of the 1970s like the Baader-Meinhof Gang in Germany and the Weather Underground in the U.S. You had a similar evolution where you had people really fired up and didn’t think protests were getting anywhere and saw that political violence was the better route.”

Supporters take part in the Pledge of Allegiance before Donald Trump’s speech during a campaign rally in Butler, Pa., on July 13, 2024.

(Evan Vucci / Associated Press)

In an age of the lone gunman, amid deepening rancor in a society inured to gun violence and militant political rhetoric, the shooting was another potent sign of what counterterrorism agencies are facing. The radicalization of potential militants “is different today with the internet and AI-generated materials,” Clarke added. “You have people imbibing propaganda all day, so moving over that [violent] line is a lot easier” than it was in the past.

Assassinations and acts of political violence are scattered throughout U.S. history. In recent times, protests and violence have been fueled by deep grievances against the government and cultural war skirmishes including Trump’s lies that he won the 2020 election — which weeks later led to an insurrection at the Capitol — book banning, gender issues and U.S. military support for Israel in the Gaza war.

What is notably unnerving, according to terrorism experts, is how politicians have embraced and fueled divisive discourse to heighten animosities. Shortly after Saturday’s assassination attempt, Sen. J.D. Vance (R-Ohio), who is on Trump’s short list of vice presidential candidates, posted on social media: “Today is not just some isolated incident. The central premise of the Biden campaign is that President Donald Trump is an authoritarian fascist who must be stopped at all costs. That rhetoric led directly to President Trump’s attempted assassination.”

Such sentiments are folded into an endless stream of provocation in a polarized nation: “It underscores the lack of trust many people have in our system,” said Amy Cooter, a terrorism expert with the Middlebury Institute of International Studies in Monterey. “It underscores the fragility of our democracy.”

Trump’s incendiary language, which is at the center of his campaign against President Biden, has created a tenor of outrage that has been taken up by right-wing extremists, white supremacists, conspiracy theorists, QAnon followers and Jan. 6 rioters, whom Trump referred to as warriors. “Trump himself is one of the key variables” in how the country has become accustomed to inflammatory politics, Cooter said.

Police snipers return fire after shots were heard while Republican presidential candidate Donald Trump was speaking at a campaign event in Butler, Pa., on July 13, 2024.

(Gene J. Puskar / Associated Press)

Other conservatives have face threats as well: An armed man threatened to kill Supreme Court Justice Brett M. Kavanaugh over his rulings on gun control and abortion, and a shooting by a left-wing radical wounded at least four people during a 2017 softball practice for Republican lawmakers in Alexandria, Va. Incidents by extremists on both sides are spun by media influencers and hyperbolic pundits into an unsettling version of two Americas where there is little room for unity or calming discourse.

It is too soon to know whether Crooks, a registered Republican who reportedly donated $15 to a liberal political organization, was part of the radical elements of either the left or right. But his act is a disturbing reminder of how America has become a seething nation since Trump’s election in 2016. The attempted assassination is giving some Trump supporters more grist for their accusations that the former president is being persecuted by the Biden administration.

“For weeks, Democrat leaders have been fueling ludicrous hysteria that Donald Trump winning re-election would be the end of democracy in America,” House Majority Leader Steve Scalise (R-La.), who was wounded in the 2017 softball attack, posted on social media. “Clearly, we’ve seen far-left lunatics act on violent rhetoric in the past. This incendiary rhetoric must stop.”

The charged atmosphere has put mounting pressure on law enforcement and Homeland Security officials as the Republican National Convention opens in Milwaukee on Monday and Democrats prepare for their convention in late August in Chicago. Officials at both sites — fearing threats including lone wolf gunmen and an array of radicals — have limited how close demonstrators can get to convention venues.

The Secret Service is under scrutiny for how Crooks, who brandished an AR-15-style gun and also had explosive devices in his car, could go undetected on a rooftop at a political rally. In announcing a plan to introduce legislation to tighten security around candidates, Reps. Mike Lawler (R-N.Y.) and Ritchie Torres (D-N.Y.) released a joint statement Sunday saying “it’s clear that more protection is needed … anything less would be a disservice to our democracy.”

“There is growing concern that this year’s elections could lead to — or even be decided by — political violence,” Garen Wintemute, professor of emergency medicine at UC Davis and director of the California Firearm Violence Research Center, wrote in the Hill in June. A survey done by the center, he noted, was “deeply concerning. In the election year of 2022, nearly one-third of participants (32.8 percent) considered violence to be usually or always justified to advance at least one political objective.”

The survey, said Wintemute, suggesting that political violence should be considered a health problem because of those it can kill or injure, found that 13.7% of those polled “strongly or very strongly agreed with the statement that ‘in the next few years, there will be civil war in the United States.’” That percentage dropped to 5.7 in the nonelection year of 2023.

Another danger also comes from so-called accelerationism, a philosophy espousing that violence is needed to overhaul societies that have gone astray. Believers in this creed come from all political backgrounds — some have no politics at all — and have been energized by extremist chat groups and twisted ideologies. Recent cases of accelerationist shooters include Brenton Tarrant, who murdered 51 Muslims in two mosques in New Zealand in 2019 and Payton Gendron, who killed 10 Black people at a supermarket in Buffalo, N.Y., in 2022.

“Acceleration is the glue” that brings disparate groups together for a goal, said Cooter, who added that people such Tarrant and Gendron leave behind manifestos and are sometimes referred to as “saints” by their followers. “It’s a scary time. The individual act of violence with the intent to inspire others.”

Over the weekend, Democrats and Republicans sought to bring down tensions. Pennsylvania Gov. Josh Shapiro, a Democrat, said: “This is a moment where all leaders have a responsibility to speak, and act, with moral clarity. Where all leaders need to take down the temperature and rise above the hateful rhetoric.”

But on talk shows and in chat rooms the grievances and hostilities that have led the nation to this point were not quieted.