WASHINGTON (AP) — A man armed with an AR-15 dies in a shootout after trying to breach FBI offices in Cincinnati. A Pennsylvania man is arrested after he posts death threats against agents on social media. In cyberspace, calls for armed uprisings and civil war grow stronger.

This could be just the beginning, federal authorities and private extremism monitors warn. A growing number of   ardent Donald Trump supporters   seem ready to strike back against the FBI or others who they believe go too far in investigating the former president.

Law enforcement officials across the country are warning and being warned about an increase in threats and the potential for violent attacks on federal agents or buildings in the wake of the FBI’s   search of Trump's Mar-a-Lago home .

Experts who study radicalization and online disinformation — such as Trump's aggressive false claims about a stolen election — note that the recent increase was sparked by a legal search of Trump's Florida home. What might happen in the event of   arrests or indictments ?

“When messaging reaches a certain pitch, things start to happen in the real world,” said former New Jersey Attorney General John Farmer, a onetime federal prosecutor who now directs the Eagleton Institute of Politics at Rutgers University. “And when people in positions of power and public trust start to echo extremist rhetoric, it’s even more likely that we’re going to see real-world consequences.”

Amplified by right-wing media, angry claims by Trump and his allies about the search are fanning the flames of his supporters' distrust of the FBI —   though it's led by a Trump appointee   — and the federal government in general. And at least a few of Trump's supporters now appear to be acting on his anger.

Last week a man wearing body armor and armed with an assault rifle and a nail gun tried to breach the FBI's Cincinnati office. He was later shot and killed by police after exchanging fire with officers. Authorities say they believe the man had   posted dark messages on Truth Social , Trump's online platform, including one that said federal agents should be killed on sight.

Another man   drove his car into a U.S. Capitol barricade   Sunday and began firing gunshots into the air before he fatally shot himself.

On Monday, the Department of Justice announced the arrest of a Pennsylvania man who had made repeated threats on the lives on FBI agents on Gab, a platform   popular with Trump supporters .

“You’ve declared war on us and now it’s open season on YOU,” he wrote in one post shared by authorities.

A joint intelligence bulletin from the FBI and Homeland Security warns about an increase in violent online threats targeting federal officials and government facilities. Those include “a threat to place a so-called dirty bomb in front of FBI headquarters,” along with calls for “civil war” and “rebellion,” according to a copy of the document obtained by The Associated Press.

Mentions of “civil war” on platforms including Facebook and Twitter increased tenfold in the hours immediately after last week's search of Mar-a-Lago, according to an analysis by Zignal Labs, a firm that analyzes social media content.

Many of the posts contained baseless claims suggesting President Joe Biden ordered the FBI to search Trump's home, or that the FBI planted evidence to incriminate Trump.

“Biden sending the FBI to raid a former President, Mr. Donald Trump’s home is a declaration of WAR against him and his supporters," wrote one poster on the Telegram platform.

The intelligence bulletin also noted federal law enforcement officials have identified multiple threats against government officials involved in the Mar-a-Lago search, including calls to kill the magistrate judge who signed the search warrant.

The names and home addresses of FBI agents and other officials have been posted online, along with references to family members who could be additional targets, according to the intelligence documents.

The threats are ominously similar to the   online rhetoric   that preceded the Jan. 6, 2021,   attack on the U.S. Capitol,   says Rep. Bennie Thompson, a Mississippi Democrat who chairs the House Jan. 6 committee and the Committee on Homeland Security.

“These threats of violence and even civil war – coming predominantly from right-wing extremists online – are not only un-American but are a threat to our democracy and the rule of law,” Thompson said.

The search of Trump's residence   was executed based on a lawfully obtained warrant signed by a judge. But that's beside the point for Trump and   his allies .

“This is an assault on a political opponent at a level never seen before in our Country," Trump wrote Monday in a post on his Truth Social. "Third World!”

Republican Rep.   Paul Gosar   of Arizona equated the investigation with “tyranny” and tweeted, “We must destroy the FBI.”

Another Arizona congressman, Republican Andy Biggs, sought to place some blame on the individual agents who executed the search. “This looked more like something you would see in the former Soviet Union," Biggs said this week. "Why did all those agents willfully go along?”

Republican Sen. John Thune told reporters in Sioux Falls, South Dakota, on Tuesday that though the Justice Department has shown it followed legal protocols in obtaining the search warrant, its reticence about the Trump investigation has caused people to question law enforcement's motives.

“There’s just a lot of unanswered questions that, left to a vacuum, create lots of suspicions among the American people, and the one thing you don’t want is people not trusting law enforcement," Thune said.

Other Republicans have tried to temper the rhetoric, as Arkansas Gov. Asa Hutchinson did during an appearance over the weekend on CNN. “We need to pull back on casting judgment on them,” Hutchinson said of the agents. ”The FBI is simply carrying out their responsibilities under the law.”

But many in the conservative media haven't heeded that advice.

“The raid on Mar-a-Lago was not an act of law enforcement, it was the opposite of that,” Tucker Carlson said on his Fox News show Monday night. “It was an attack on the rule of law.”

Fox also shared a   doctored photo   that falsely depicted the judge who signed the warrant receiving a foot massage from Ghislaine Maxwell. Maxwell was sentenced in June to 20 years for helping her boyfriend Jeffrey Epstein abuse underage girls. The original photo was not of the judge but of Epstein, who committed suicide in 2019 while awaiting trial. Fox News’ Brian Kilmeade later said the doctored image was shared as a joke.

The roots of Republican anger at the FBI go back to the 2016 election and investigations of the   Trump campaign's alleged ties with Russia   and of   Hillary Clinton's handling of classified material   in a private email account. That fury has only increased as new investigations focus on Trump, his efforts to overturn the 2020 election and his handling of classified material since leaving office.

Baseless claims that the FBI   secretly framed Trump supporters   for their violent actions on Jan. 6 also stoked the ire of conservative social media users.

“Well guys you started this civil war," wrote one poster on Gab “And others are going to sure end it for you.”

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More on Donald Trump-related investigations: apnews

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Associated Press writers Michael Balsamo and Stephen Groves contributed to this report.