An armed man wearing body armor was slain after attempting to “breach” an FBI headquarters in Ohio authorities report.
The suspect fled the scene in Cincinnati and was shot by police after an hours-long standoff in a rural area.
Authorities said they are investigating if the guy, identified by media as 42-year-old Ricky Shiffer, had any connections to far-right organizations.
The head of the FBI said violence and threats against the agency “should be deeply concerning to all Americans”.
Police have not formally identified the suspect killed in Ohio on Thursday, and did not comment on his motive during news briefings.
Unnamed law enforcement officials told US media the suspect may have been present at the Capitol building in Washington on the day of last year’s riot by Trump supporters, although he was not charged with any crimes in connection to the disorder.
The Ohio incident comes as US officials warn of increased threats against law enforcement after the FBI executed a search warrant on Monday at former President Donald Trump’s Mar-a-Lago estate in Florida.
Police said the suspect tried unsuccessfully to breach a visitor security screening area at the FBI office in Cincinnati at around 09:15 (13:15 GMT).
He fled the area, but was spotted about 20 minutes later by a police officer, Ohio State Highway Patrol spokesman Nathan Dennis told a news conference.
A chase ensued until the suspect stopped and exchanged gunfire with officers before fleeing into a cornfield.
After an hours-long standoff, he raised a weapon toward officers and was killed by police around 15:00 local time, said, Mr. Dennis. No police were injured in the shootout.
According to NBC News, the man fired a nail gun at the FBI building and was also armed with a semi-automatic rifle.
Most of the tweets were about the 2020 election, saying it had been stolen from Mr. Trump. The posts also included calls for violence against Democrats, the FBI, and the Supreme Court.
At least two posts on the Twitter account – including one saying “I was there” – suggest the person attended the Capitol riot.
On Truth Social, the website owned by Mr. Trump, an account in the name of one Ricky Shiffer posted earlier on Thursday, calling for Americans to “be ready to combat”, adding: “I am proposing war.”
One post appeared to have been made after the incident at the FBI office in Cincinnati.
FBI Director Christopher Wray, who was appointed by Mr. Trump in 2017, said in a statement that “unfounded attacks on the integrity of the FBI erode respect for the rule of law and are a grave disservice to the men and women who sacrifice so much to protect others.
In a speech to FBI field agents in the state of Nebraska on Wednesday, he called online threats to officials “deplorable and dangerous”, adding: “Violence against law enforcement is not the answer, no matter who you’re upset with.”