Tyler Pager reported from Washington and Nick Corasaniti from New York.
President Trump and his top advisers are escalating their attacks on their opponents in the wake of Charlie Kirk's killing, placing the blame for political violence on Democrats alone and signaling a broad crackdown on critics and left-leaning institutions.
Mr. Trump blamed the "radical left" almost immediately after Mr. Kirk was shot, before the authorities had identified a suspect. He promised to find those responsible for political violence, as well as the "organizations that fund it and support it."
Mr. Trump has an expansive view of those he deems radical, applying that term to almost all of his political adversaries. In his second term, Mr. Trump has pushed the boundaries of his authority to exact retribution on political opponents and institutions.
The death of Mr. Kirk, a popular young conservative activist, has added fuel to Mr. Trump's campaign against his opponents. He and his administration have promised to bring the killer to justice while using the moment to blame the left -- and only the left -- more broadly.
Critics of the administration now worry that Mr. Kirk's murder could be used as a pretext to move even more aggressively against those who speak out against Mr. Trump.
The authorities were still working to discern a motive in the killing on Saturday morning. The suspect had recently spoken with a family member about the fact that Mr. Kirk was going to hold an event in Utah, according to a police affidavit, and he and his relative discussed "why they didn't like him and the viewpoints he had."
America in recent years has seen a wave of violence across the political spectrum, targeting Democrats and Republicans, but Mr. Trump has focused only on attacks against conservatives and his allies. On Friday, he appeared to excuse right-wing radicals by arguing they were motivated by a desire to reduce crime.
And while the president has provided few specifics about how he plans to address rising political violence or mete out punishments, several administration officials vowed to scrutinize speech by those who have denounced Mr. Kirk -- a self-declared supporter of free speech -- and his often inflammatory views.
Defense Secretary Pete Hegseth said that his agency was closely tracking any military personnel who celebrated or mocked Mr. Kirk's death, and Christopher Landau, the deputy secretary of state, suggested the administration would strip visas from individuals who celebrated Mr. Kirk's death.
"I want to underscore that foreigners who glorify violence and hatred are not welcome visitors to our country," Mr. Landau wrote on X.
On Capitol Hill, Representative Clay Higgins, Republican of Louisiana, said he would use his congressional authority to seek immediate bans for life from social media platforms for anyone who "belittled the assassination of Charlie Kirk."
"I'm also going after their business licenses and permitting, their businesses will be blacklisted aggressively, they should be kicked from every school, and their drivers licenses should be revoked," he wrote on X. "I'm basically going to cancel with extreme prejudice these evil, sick animals who celebrated Charlie Kirk's assassination. I'm starting that today."
Mr. Trump also renewed his call on Friday for prosecutors to file racketeering charges against George Soros, one of the Democratic Party's biggest donors. Mr. Trump and his allies have long claimed without evidence that Mr. Soros foments violent protests.
"We're going to look into Soros because I think it's a RICO case against him and other people because this is more than protests," he said on Fox News. "This is real agitation; this is riots on the street -- and we're going to look into that."
A spokesman for Mr. Soros's organization, Open Society Foundations, denied the allegations and called the threats "outrageous."
Stephen Miller, a top adviser to the president, characterized the current moment in America as a battle between "family and nature" and those who celebrate "everything that is warped, twisted and depraved."
He said the "fate of millions depends upon the defeat of this wicked ideology."
In an interview on Thursday with Scott Jennings, a conservative radio host, Susie Wiles, the White House chief of staff, said that before Mr. Kirk's death, the administration had been working on "a more comprehensive plan on violence in America, the importance of free speech and civil speech," though she did not provide any details. White House officials also declined to answer questions about the plan.
Experts warn that the polarization in the country is growing increasingly dire.
"We're watching grief, anger, blame and calls for retribution all occurring in parallel and all occurring in the public sphere," said Sean Westwood, an associate professor of government at Dartmouth College and the director of the Polarization Research Lab.
He added: "In that kind of environment, the loudest voice is going to prevail, and in the moment, that loudest voice is calling for further division. So unlike the past, where we've had leaders pushing us together, we now have leaders who are pushing us apart, and that could lead to more violence."
Gov. Spencer Cox of Utah, a Republican, took a strikingly different approach during the news conference where officials announced the arrest of a suspect on Friday. Mr. Cox asked for people to lower the political temperature and stay off social media.
"We can return violence with violence, we can return hate with hate, and that's the problem with political violence -- is it metastasizes," he said. "Because we can always point the finger at the other side. And at some point, we have to find an off-ramp, or it's going to get much, much worse."
Liberal organizations fear that the Trump administration will use the shooting to justify a crackdown on their operations, targeting their cash flow, nonprofit status or contributors.
Jess O'Connell, a political strategist who co-founded the Democracy Security Project, said left-leaning civil and nonprofit organizations had been grappling with heightened security threats since Mr. Trump took office. But the president's explicit calls to crack down on left-wing activists has dramatically escalated those fears, she said.
"The president has been looking for anything he can use to justify a big crackdown on his perceived political enemies that includes not just nonprofits but civic and cultural organizations," she said. "It's a danger to all of us when the president picks sides on who we should mourn."
Sean Kennedy, a conservative activist who researches left-wing donors at the Law Enforcement Legal Defense Fund, said some scrutiny of liberal money was justified after Mr. Kirk's shooting -- but not all.
"Trump is right to investigate and prosecute left-wing purveyors of political violence," he said. Still, he added that "however abhorrent their ideas may be, George Soros isn't Al Capone and the A.C.L.U. isn't antifa."
In Utah on Thursday, officials arrested a 22-year-old man, Tyler Robinson, in connection with Mr. Kirk's killing. Investigators said they had found messages inscribed on unfired cartridges in the woods near campus alongside the rifle that had been used in the attack. The messages, they said, suggested familiarity with antifascist symbolism and the irreverent slang of internet memes and role-play communities.
Missing from Mr. Trump's denunciations of his opponents after Mr. Kirk's death is any mention of political violence that has targeted Democrats.
Melissa Hortman, the former Democratic speaker of the Minnesota House of Representatives, was killed in June; Gov. Josh Shapiro of Pennsylvania was the victim of an arson attack on his home in April while he and his family slept; Paul Pelosi, the husband of former Speaker Nancy Pelosi, was violently beaten inside his home in 2022 by an intruder who was targeting Ms. Pelosi; and 13 men were arrested in 2020 for plotting to kidnap Gov. Gretchen Whitmer of Michigan.
And Mr. Kirk was far from the first prominent Republican targeted: Mr. Trump survived two assassination attempts during the presidential campaign; in 2017 Representative Steve Scalise Republican of Louisiana was injured in a shooting.
After Mr. Kirk was killed Mr. Trump ordered all flags to be lowered to half-staff said he would award Mr. Kirk the Presidential Medal of Freedom highest civilian honor.
Shane Goldmacher Lisa Lerer Theodore Schleifer contributed reporting.