The attacks on Mr. Piro, and his angry rebuttal of them, are ‌e‌mblematic of a toxic dynamic that is increasingly central to Republican Party politics. Mr. Trump’s supporters — among them, Republicans poised to take over the House next month — have seized on the letter’s accusations and stepped up their assaults on the F.B.I., seeking to undermine the bureau just as it has assumed the lead in an array of investigations of Mr. Trump.

Representative Jim Jordan, the Ohio Republican who will be the Judiciary Committee’s chairman next month, has pledged to investigate what he describes as the politicization of the F.B.I. as well as that of the Justice Department. In a taste of what is to come, the committee’s Republican staff released a 1,000-page report last month that asserted that the F.B.I. hierarchy “spied on President Trump’s campaign and ridiculed conservative Americans” and that the “rot within the F.B.I. festers in and proceeds from Washington.”

Historically, the F.B.I.’s most vocal critics have come from figures on the left, who have accused it of using heavy-handed tactics in investigating groups like trade unionists or civil rights activists. Conservatives and Republicans have, at least by tradition, supported the F.B.I. and other law-enforcement agencies.

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A majority of the attacks laid out in the Suspendables’ letter to Mr. Wray, who was appointed by Mr. Trump, echoed those by the Judiciary Committee. The panel’s report also condemned the bureau for using counterterrorism tactics to investigate conservative parents at school board meetings — an allegation that seemed to have come from a mischaracterization of the F.B.I.’s plan to track threats of violence against school board officials.

The report further accused the agency of “helping Big Tech to censor Americans’ political speech” — a claim that misrepresented the way the F.B.I. has sought for years to curb online disinformation, especially when it comes from foreign actors. Long before the House report or the letter to Mr. Wray was released, Mr. Trump and his allies in Congress and the news media were already targeting federal law enforcement officers and demonizing those who scrutinized the former president.

A group of former agents sent a letter to Christopher A. Wray, the F.B.I. director, claiming misconduct and political bias at the bureau. Stefani Reynolds for The New York Times

The attacks began after federal agents searched the office of Michael D. Cohen, Mr. Trump’s personal lawyer, for evidence of campaign finance violations. After the search, Rudolph W. Giuliani, another lawyer close to Mr. Trump, went on the warpath. He declared that the F.B.I.’s office in New York — with which he had worked closely as the U.S. attorney in Manhattan — had behaved like “storm troopers” in conducting the raid.

Since then, Mr. Trump and his supporters have gone after the bureau for its role in investigating his campaign’s ties to Russia; for purportedly failing to investigate issues surrounding Hunter Biden’s laptop; and for using informants to infiltrate a group of militiamen charged in a plot to kidnap Gov. Gretchen Whitmer of Michigan.

Some critics, including former agents, have attacked the F.B.I. for pursuing those in the mob that stormed the Capitol on Jan. 6, 2021, describing the criminal prosecution of the rioters as political persecution.

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This drumbeat of vitriol has created a reflexive reaction against the F.B.I. as nefarious and partisan among large swaths of the right, even as Mr. Trump has lost a measure of political support.

“The FBI is the largest criminal gang in the country,” the right-wing commentator Dinesh D’Souza recently wrote on Twitter, adding, “It’s America’s version of the KGB or the Chinese state police.”

To be sure, the F.B.I. has made several grievous errors in recent years. It failed to follow up on a tip that might have prevented a school shooting in Parkland, Fla., in 2018. It bungled an investigation in 2015 into claims that a doctor for U.S.A. Gymnastics had sexually abused young women.

In 2020, an F.B.I. lawyer pleaded guilty to doctoring an email that was used in preparation to ask a court to renew an order to wiretap a former Trump campaign adviser. Questions have also been raised about whether the bureau, which is in charge of preventing terrorism, could have done more to stop the Capitol attack with the use of secret informants it had within two of the far-right groups involved in the riot.

Some recent attacks on the F.B.I. by right-wing officials and figures in the news media seem intended to make money. Representative Marjorie Taylor Greene of Georgia sells “Defund the F.B.I.” baseball hats; Sebastian Gorka, a former Trump adviser, peddles T-shirts reading, “F.B.I.: Fascist Bureau of Intimidation.”

The barrage of messaging comes as the bureau itself has faced violence.

In August, an Ohio man, enraged by the search of Mar-a-Lago, tried to break into the F.B.I.’s field office near Cincinnati and was ultimately killed in a shootout with the local police. Investigators later discovered social media posts he had written encouraging others to kill federal agents.

On Dec. 16, a Tennessee man who was facing charges of assaulting the police during the Capitol attack was charged again with plotting to assassinate several of the federal agents who had investigated him. He was also accused of planning an attack on the F.B.I.’s field office in Knoxville, Tenn.

The group of former agents, calling themselves the Suspendables, spread false claims about George Piro, a highly decorated F.B.I. agent. Joe Raedle/Getty Images

The Suspendables’ letter and the House Republicans’ report were both apparently drawn from statements by former F.B.I. agents who left the bureau under a cloud and then came forward as self-described whistle-blowers. Among them is Steven Friend, a former agent from Florida, who refused to take part in a SWAT raid this summer of a Jan. 6 suspect facing arrest on misdemeanor charges.

“I have an oath to uphold the Constitution,” Mr. Friend, a 12-year veteran of the bureau, told his supervisors when he declined to join the raid on Aug. 24 in Jacksonville, Fla., which he deemed an excessive use of force. “I have a moral objection and want to be considered a conscientious objector.”

According to Justice Department records, there was only one Jan. 6-related arrest in the Jacksonville area on Aug. 24: that of Tyler Bensch, who was accused of being a member of a right-wing militia group connected to the Three Percenter movement.

What Mr. Friend omitted from his account — which was published in The New York Post and widely shared online — was that while Mr. Bensch was charged with only misdemeanors, documents in his case say that on Jan. 6, 2021, he posted a video of himself outside the Capitol wearing body armor and a gas mask and carrying an AR-15-style rifle. The documents also say that witnesses told the F.B.I. they had seen photographs of Mr. Bensch carrying a similar rifle at other times.

Former F.B.I. agents who have served on SWAT teams said the use of tactical agents during arrests has nothing to do with the charges someone is facing but is based instead on a risk assessment of the suspect.

“When in doubt, you use SWAT,” said Robert D’Amico, who served on the Miami tactical team for four years and the hostage rescue team for almost two decades. “You never let the charges dictate the tactics.”

Last year, two F.B.I. agents were killed and three more were wounded while serving a search warrant in a case involving child pornography in Florida. The suspect did not have a violent history and had been deemed low risk. The deadly episode illustrated the dangers of serving warrants or arresting suspects considered not threatening. The agents worked for Mr. Piro.

Mr. Friend’s lawyer said his client objected to the SWAT team arrest because he wanted to “de-escalate” the situation and avoid what he described as another Ruby Ridge — a reference to a botched F.B.I. raid on a white supremacist compound in Idaho in 1992 that has becoming a rallying cry for far-right extremists.

The F.B.I. declined to comment on the attacks against Mr. Piro, but three former and current law enforcement officials, speaking on the condition of anonymity to discuss personnel matters, said he was not under investigation when he retired from the bureau.

Mr. Piro said he was depressed by how some former agents had turned on the bureau.

“I am saddened by their behavior and their total disregard for those who are working for the F.B.I.,” he said, “and those who came before them to make the F.B.I. the premier law enforcement agency in the world.”

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Adam Goldman reports on the F.B.I. and national security from Washington, D.C., and is a two-time Pulitzer Prize winner. He is the coauthor of “Enemies Within: Inside the NYPD's Secret Spying Unit and bin Laden's Final Plot Against America.”

Alan Feuer covers extremism and political violence. He joined The Times in 1999.

A version of this article appears in print on Dec. 29, 2022, Section A, Page 20 of the New York edition with the headline: Investigating Trump, F.B.I. Takes Pounding From G.O.P.’s Barrages.