A single bullet killed conservative activist Charlie Kirk on Wednesday and pierced the collective conscience of Americans amid an ongoing era of political violence.
"It makes you wonder what is going on in our country. That this kind of violence is so prevalent," said Sen. Susan Collins (R-ME).
"We just cannot be a country that turns to violence when we disagree with people," said Sen. Elizabeth Warren (D-MA).
But more and more, that's been the case in the United States.
Last year, two assassination attempts targeted then-candidate President Trump.
In April, Pennsylvania's Democratic Governor Josh Shapiro's house was set on fire during the Jewish holiday of Passover.
In June, a gunman assassinated Democratic Minnesota state Representative Melissa Hortman and her husband, and shot and injured another state lawmaker.
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Researchers say the rise in political violence is driven in part by polarization.
"When you get a lot of people who think alike talking to one another, they become more extreme," said Professor Clark McCauley.
McCauley says some fringe actors feel a moral pull to act on behalf of a cause.
"It's the moral judgment that makes it possible. The enemy, the threat is not just wrong, they're a little less than human," McCauley said.
The new reality is changing how politicians are thinking about public events.
"We've got to protect people who run for public office, or no one will," said U.S. House Speaker Mike Johnson (R-LA).
"An open-door event like those are very hard to defend. A lot of vulnerabilities," Gene Petrino, a retired SWAT commander, told Scripps News.
He expects the Kirk assassination will change how law enforcement secures political events.
"I think overall we're gonna see a reduction and I think we're also gonna see additional security measures as we move forward," Petrino said.
