An F.B.I. agent on Wednesday fatally shot a person in Provo, Utah, who officers mentioned was armed and had threatened to assassinate President Biden simply hours earlier than the president was scheduled to talk in close by Salt Lake City.

Craig D. Robertson, 75, was additionally charged with threatening to shoot different elected officers, together with Alvin L. Bragg, the Manhattan district legal professional, in addition to with making threats towards legislation enforcement officers, in line with court documents filed a day earlier in U.S. District Court in Salt Lake City.

The capturing comes at a second of intense polarization in American politics. The three indictments of former President Donald J. Trump have provided fodder for supporters and allies, who’ve seized on his mounting authorized peril to fan a story of a Justice Department weaponized towards him and bent on derailing the Republican front-runner’s marketing campaign to retake the White House.

The F.B.I. mentioned in a press release on Wednesday afternoon that it was investigating an “agent-involved shooting” that had occurred round 6:15 a.m. in Provo whereas brokers had been attempting to serve arrest and search warrants at a house.

A federal legislation enforcement official, talking on the situation of anonymity to debate the capturing, mentioned that Mr. Robertson was armed on the time. Officials provided few different particulars.

A White House official mentioned that Mr. Biden had been briefed on the matter, and a girl who was listed in public data as Mr. Robertson’s daughter declined to remark when reached by cellphone.

According to the court docket submitting, Mr. Robertson owned quite a few firearms, together with a sniper rifle. The criticism additionally laid out his historical past of threats on social media, the place he referred to his weapons as Democratic eradicators. In one put up final yr, he photographed three rifles and mentioned he was “getting ready for the 2024 election cycle.” He repeatedly taunted the brokers investigating him, saying they got here near “violent eradication.”

Last week, he famous on social media that Mr. Biden could be visiting Utah and that he was going to “dust off” an M24 rifle and get out his previous camouflage swimsuit, one sometimes utilized by snipers.

According to the criticism, different on-line posts indicated Mr. Robertson’s intent to kill Mr. Biden and Mr. Bragg, who’s main a prosecution of Mr. Trump in New York in reference to a hush cash fee to a porn star throughout his 2016 marketing campaign.

On Sept. 19, 2022, Mr. Robertson wrote on Facebook that “the time is right for a presidential assassination or two.”

“First Joe then Kamala,” he wrote. In one other message, he declared that he needed shoot Mr. Bragg within the head and watch him die.

The put up additionally talked about George Soros, the financier and Democratic megadonor, who has been a goal of Mr. Trump and Republicans in Congress, and echoed Mr. Trump’s assaults on Mr. Bragg and Mr. Soros.

Other topics of Mr. Robertson’s threats: Attorney General Merrick B. Garland, whom he described as a Nazi; Gov. Gavin Newsom of California; and Letitia James, the New York legal professional normal.

In September 2022, referring to Ms. James, Mr. Robertson wrote on Facebook that “a sniper’s bullet does not recognize your qualified immunity.” In October 2022, he shared an image of a semiautomatic handgun, calling it a “Merrick Garland eradication tool.”

In one more Facebook put up in October, Mr. Robertson described a “patriotic dream” during which he mentioned stood over a wounded Mr. Newsom, “my suppressed S&W M&P 9mm still smoking.”

In a Facebook put up in March, Mr. Robertson taunted the F.B.I.

“To my friends in the Federal Bureau of Idiots: I know you’re reading this,” he wrote.

Spokespeople for Mr. Bragg and Ms. James declined to remark.

The F.B.I. has been more and more involved about threats to its brokers. Last yr, legislation enforcement officers shot and killed a person who they mentioned tried to interrupt into the F.B.I.’s Cincinnati workplace.

And a Tennessee man accused of assaulting the police throughout the Jan. 6 assault on the Capitol confronted further prices of plotting to assassinate several of the federal agents who had investigated him. He was additionally accused of planning an assault on the F.B.I.’s subject workplace in Knoxville, Tenn.

Mr. Trump and his supporters have downplayed the notion that his incendiary criticisms of the Justice Department and the F.B.I. are something apart from an expression of respectable grievances.

But federal and native authorities have remained on heightened alert, offering further safety to prosecutors investigating Mr. Trump and the judges presiding over his circumstances.

Zolan Kanno-Youngs and Jonah E. Bromwich contributed reporting. Kitty Bennett contributed analysis.