When a Donald Trump supporter was killed by police after shooting up an FBI office in the US Midwest, investigators turned to the assailant's social media to confirm a motive they already feared.

It turned out he had discussed leading an "insurrection against the people who usurped our government" in revenge for the bureau's raid on the former president's Florida home in August.

"I am proposing war," read one post from his since-deleted account on Trump's Truth Social platform, US media reported. "Kill the FBI on sight."

In the 21 months since a pro-Trump mob ransacked the US Capitol, violent rhetoric and the attacks it foments have become an increasingly worrying contusion on the US body politic, analysts warn.

Many experts are bracing for a spike in the already burgeoning discourse around "civil war" as the November 8 midterm elections approach and campaign rhetoric becomes more bellicose.

Twitter mentions of the phrase skyrocketed in the hours after the FBI's search for mishandled government secrets at Trump's beach club -- from 500 tweets an hour to 6,000, according to The New York Times.

It is often used hyperbolically to refer to bitter partisanship, but many of the mentions were literal calls to arms, envisaging war, or years of insurgency.

A YouGov poll in August showed that 54 percent of self-identified "strong Republicans" believe a civil war was at least "somewhat likely" in the next decade.

The slice of American society holding this view includes the founder of anti-government militia the Oath Keepers, who told his cadres two days after Trump's election defeat: "We aren't getting through this without a civil war."

'Liberate Michigan!'

K. Campbell, a military intelligence veteran who has led security risk assessments for the US government, is most concerned by the threat from groups such as the Oath Keepers.

"In the 1970s to the early 1980s, left-wing groups were the highest threat inside the US. The bombing of the US Capitol in 1983 by America's only female terrorist group is one example," he told AFP.

"But over the past several years, far-right groups have been responsible for the majority of attacks in the US, including against law enforcement."

Trump has regularly invoked the specter of unrest when facing some political or legal defeat.

His role in stoking anger on the day of the US Capitol riot has been widely discussed, but it is often forgotten that it was his second such transgression in months.

The previous April, he urged activists to "Liberate Michigan!" among a series of incendiary posts critical of the Midwestern state's Covid-19 restrictions.

Days later, heavily armed protesters occupied the state capitol.

Members of Trump's own cabinet, not to mention the Republican leader in the Senate and the House Democrats who impeached him, blamed the ex-president for the January 6, 2021, riot in Washington.

They are backed by numerous studies demonstrating a causal link between aggressive rhetoric and political violence.

Robert Pape of the University of Chicago has conducted several polls since the Capitol assault, demonstrating that between 15 and 20 million Americans believe violence would be justified to return Trump to office.

Republicans, however, accuse Democrats of overreacting to figurative language and ignoring leftist aggression such as harassment of conservative Supreme Court justices and the 2017 mass shooting at a congressional sports event.

9,600 threats in a year

Still, law enforcement agencies say right-wing violence is the larger threat.

Discourse that was once taboo is now commonplace on the far right, with Republican flamethrowers in Congress incorporating violent language and imagery into their stump speeches.

Threats against members of Congress reached a record high of 9,625 in 2021, according to data provided by the Capitol Police, compared with just 3,939 five years ago.

This year, a progressive Democratic lawmaker has had an armed man show up outside her home. Another livestreams videos of people harassing her outside the Capitol.

Conservatives in the House and Senate have been targeted by death threats and vandalism of their homes. One survived an attempted stabbing.

Michael Susong, a former CIA officer who was decorated for heroism in the field, says there is "no credible threat" of civil war, although he acknowledges that political discourse has "slipped the bounds of decency."

"Social media platforms and pervasive real-time global communications can exacerbate real and perceived grievances," said Susong, now the senior vice president for global intelligence at risk management company Crisis24.

"This accelerates the radicalization of individuals who internalize extreme narratives of events or history. Most times the result is uncivil discourse and hollow threats. But we see, and will always see, a few individuals provoked to violence."

© Agence France-Presse