A Justice Department inspector general’s report examining the FBI investigation of President Trump’s 2016 campaign rebutted conservatives’ accusations that top FBI officials were driven by political bias to illegally spy on Trump advisers but also found broad and “serious performance failures” requiring major changes.

The 434-page report issued Monday by Justice Department Inspector General Michael Horowitz concluded that the FBI had an “authorized purpose” when it initiated its investigation, known as Crossfire Hurricane, into the Trump campaign. In doing so, Horowitz implicitly rejected assertions by the president and fellow Republicans that the case was launched out of political animus or that the FBI broke its own rules on using informants.

As the probe went on and the FBI sought court approval to surveil a former campaign aide, officials repeatedly emphasized damaging information they heard about Trump associates and played down exculpatory evidence they found, the report said. The bureau promptly indicated that it would implement dozens of corrective measures in response to Horowitz’s report and that disciplinary action remains a possibility.

Conservatives and liberals alike claimed victory after the report’s release — with Republicans asserting that it exposed serious wrongdoing while Democrats said it validated the Russia investigation. Trump called the report’s findings “far worse than anything I would have imagined.”

“This was an overthrow of government,” he said. “This was an attempted overthrow, and a lot of people were in on it, and they got caught, they got caught red-handed.”

The report, which was based on more than 1 million documents and more than 170 interviews, is the most exhaustive assessment to date of the investigation that roiled Trump’s presidency and would ultimately be overtaken by special counsel Robert S. Mueller III. But in the minutes after the document was released, it became clear that the report would not be the last word.

In a statement, Attorney General William P. Barr disagreed with one of the inspector general’s key conclusions, saying the FBI launched an investigation of a presidential campaign “on the thinnest of suspicions that, in my view, were insufficient to justify the steps taken. It is also clear that, from its inception, the evidence produced by the investigation was consistently exculpatory.”

So, too, did Connecticut U.S. Attorney John Durham, whom Barr handpicked to conduct an investigation similar to that of Horowitz. “Our investigation has included developing information from other persons and entities, both in the U.S. and outside of the U.S.,” Durham said in a statement. “Based on the evidence collected to date, and while our investigation is ongoing, last month we advised the Inspector General that we do not agree with some of the report’s conclusions as to predication and how the FBI case was opened.”

Durham’s investigation is ongoing — and what evidence he has to rebut Horowitz remains unclear. Some involved in the Russia case said they were mystified by his statement.

A draft of the inspector general’s report contained a footnote saying Horowitz had asked Durham whether he had evidence to support a right-wing theory that the investigation began as a setup by U.S. intelligence using a Maltese professor to entrap the Trump campaign, according to people who saw the draft. Durham said he did not, the people said. That note about the probe’s origins, though, does not appear in the publicly released version of the document. The report has some redactions, but it is unclear if that section was removed or blacked out.

Speaking Monday on MSNBC, former FBI director James B. Comey took aim at Durham, saying he “can’t make any sense” of Durham’s statement disagreeing with the inspector general and predicting that if the prosecutor does not find information to satisfy Trump, someone will be assigned to investigate him.

“Don’t be a part of the sliming of the IG and the department as a whole,” Comey said. “Do your work.”

Politicians of both parties seized on various portions of the report that supported their long-held views on the Russia probe.

Sen. Richard Blumenthal (D-Conn.) said the inspector general had “completely demolished” some of conservatives’ assertions about the origins of the probe, though investigators did find problems. Sen. Dianne Feinstein (D-Calif.) noted that Trump had called the case a “hoax” and a “witch hunt,” and she said the report “shows that simply isn’t the case.”

Rep. Mark Meadows (R-N.C.), a strong Trump ally, meanwhile, said, “This is not a good day for the FBI.”

“There’s more than enough evidence in this report that would suggest a major overhaul in terms of policies and procedures,” he said.

Rep. Devin Nunes (R-Calif.) said the inspector general’s report showed that a previous assessment, authored by Republicans on the House Intelligence Committee, of the FBI’s surveillance activities directed toward the Trump campaign “was accurate and actually understated the FISA abuse the dirty cops engaged in.” FISA refers to the Foreign Intelligence Surveillance Act.

Representatives for those who led the probe and who have become frequent targets of attacks from the president said Horowitz’s findings vindicated their clients.

Michael R. Bromwich, a lawyer for former FBI deputy director Andrew McCabe, noted that the inspector general “found an ample factual basis for the launching of the Russia investigation” and that those errors that were documented “took place at multiple layers below” McCabe.

“Mr. McCabe did not know about any of these issues, and the report makes clear that there is no way he could have known unless someone specifically brought them to his attention,” Bromwich said. “The report makes clear that no one did.”

Aitan Goelman, a lawyer for former FBI deputy assistant director Peter Strzok, said, “This independent Department of Justice report confirms what everyone who has worked with Pete Strzok already knew — in 25 years of protecting our country, his personal opinions never impacted his work as an official of the FBI.”

“Those who recklessly deny this truth for political gain do a grave disservice,” he said.

Horowitz was particularly critical of applications the FBI made to the Foreign Intelligence Surveillance Court to monitor former Trump campaign adviser Carter Page, asserting that those applications contained “significant inaccuracies and omissions” and that agents “failed to meet the basic obligation” to ensure the applications were “scrupulously accurate.”

The inspector general found that “so many basic and fundamental errors were made by three separate, hand-picked teams on one of the most sensitive FBI investigations that was briefed to the highest levels within the FBI,” he concluded that there was a failure of “not only the operational team, but also of the managers and supervisors, including senior officials, in the chain of command.”

Among those failures was the growing body of information the FBI had gathered suggesting that former British spy Christopher Steele — whose reports were a key element justifying the surveillance of Page — was not as reliable a source as officials had described to the court.

“As the FBI obtained additional information raising significant questions about the reliability of the Steele election reporting, the FBI failed to reassess the Steele reporting relied upon in the FISA applications, and did not fully advise” Justice Department officials of those issues.

As a result of those findings, Horowitz has launched a broader audit of FISA work, to study how systemic those problems may be for FISA applications in counterintelligence and counterterrorism cases. He did not, however, conclude that the bureau’s applications to monitor Page should have been rejected.

“We do not speculate whether the correction of any particular misstatement or omission, or some combination thereof, would have resulted in a different outcome,” the report said. “Nevertheless, the Department’s decision-makers and the court should have been given complete and accurate information so that they could meaningfully evaluate probable cause before authorizing the surveillance of a U.S. person associated with a presidential campaign.”

The FBI also used informants to surreptitiously record conversations with Trump associates, including one person described in the report as “a high-level Trump campaign official who was not a subject of the investigation.”

Agents used informants, also known as confidential human sources, “to interact and consensually record multiple conversations with Page and [former Trump campaign adviser George] Papadopoulos,” the report said.

While the inspector general concluded that the FBI followed its procedures regarding investigations, it also recommended that in the future, such sensitive investigations involving major presidential campaigns be more closely supervised by the Justice Department. FBI officials told the inspector general that they heard political information in the course of their work, though one senior official said they considered it “extra crap.”

FBI Director Christopher A. Wray said the bureau accepts the findings and plans to make a host of changes, including to how it gathers and submits information for surveillance applications.

“I have ordered more than 40 corrective steps to address the Report’s recommendations,” Wray said in a statement. “Because our credibility and brand are central to fulfilling our mission, we are also making improvements beyond those recommended,” adding that the FBI “will not hesitate to take appropriate disciplinary action if warranted at the completion of the required procedures for disciplinary review.”

He noted to ABC News, though, that it was “important that the inspector general found that, in this particular instance, the investigation was opened with appropriate predication and authorization.”

The report also took aim at the FBI’s handling of a key piece of information about Page: that he had previously provided information to another U.S. agency. The report doesn’t identify the agency, but people familiar with the matter said it was the CIA.

Though the agency told the FBI in August that it had a relationship with Page, including in the time frame that the FBI found Page was acting suspiciously, that information was left out of the FISA application, the report said.

In preparation for an application to the FISA court to continue surveillance on Page, FBI lawyer Kevin Clinesmith forwarded to an FBI agent a prior email from a CIA liaison, but Clinesmith added the words “not a source” to the liaison’s email, making it appear that the liaison had denied that Page was a source for the CIA, the report said. That conduct is now being investigated as a possible crime by Durham, the U.S. attorney in Connecticut, according to people familiar with the matter.”

The inspector general also found instances of Page and Papadopoulos making exculpatory statements to FBI informants, but those were not provided to the lawyers overseeing the Page surveillance. At one point, Papadopoulos told an FBI informant that the Trump campaign “of course, [does not] advocate for this type of activity because at the end of the day it’s . . . illegal” and that “our campaign is not . . . engag[ing] or reaching out to WikiLeaks or to the whoever it is to tell them please work with us, collaborate because we don’t, no one does that.” The anti-secrecy group WikiLeaks obtained and published stolen emails damaging to Hillary Clinton’s campaign, a subject of intense interest within Trump’s campaign.

Another key finding by the inspector general is that the FBI used a national security briefing to the Trump campaign in August 2016 to try to further its collusion investigation. The FBI provides intelligence briefings to major-party candidates to warn them about the risks posed by foreign intelligence operatives, but in this case, the briefing had a secondary, unstated purpose: to put an agent working on the Russia collusion investigation in the room with Trump national security adviser Michael Flynn and report back on what ­Flynn said.

“The FBI’s use of this briefing for investigative reasons could potentially interfere with the expectation of trust and good faith among participants in strategic intelligence briefings,” the report concluded.

Horowitz is scheduled to discuss his findings Wednesday during a public hearing before the Senate Judiciary Committee.

Rosalind S. Helderman and Tom Hamburger contributed to this report.

Correction: A previous version of this story stated incorrectly that an FBI lawyer sent an altered email to a Justice Department lawyer. The altered email was sent to an FBI agent.