The FBI had barely closed a politically volcanic investigation into Secretary of State Hillary Clinton’s use of a private email server when it got a troubling tip about her rival’s presidential campaign.

On July 28, 2016, the bureau received information from an Australian diplomat, who said a Donald Trump campaign aide had “suggested the Trump team had received some kind of suggestion from Russia” that Moscow could anonymously release damaging information about Clinton, according to the long-awaited Justice Department inspector general’s report released Monday.

The tip, vague as it was, shook senior FBI officials, who were already investigating suspected Russian interference in the 2016 campaign, including the theft of emails from the Democratic National Committee. Three days later, the FBI took the momentous decision to open a counterintelligence investigation of a presidential campaign, as the election season entered the home stretch.

Almost ever since, the origin of that investigation has been a lens through which Trump’s most ardent supporters and critics see his entire presidency. To the former camp, it is the original sin of a “deep state” determined to prevent Trump’s election on the basis of slim, if not outright fabricated, evidence. To Trump’s opponents, the investigation was the only possible response to the threat of a foreign adversary compromising the Republican presidential nominee’s campaign.

The 434-page report by Inspector General Michael Horowitz will not settle the feud. It finds that the FBI’s investigation, dubbed Crossfire Hurricane, was sufficiently predicated and largely confirms previous findings about the origins of the probe by special counsel Robert S. Mueller III.

But it also questions officials’ investigative decisions, notably about how they decided to secretly monitor the communications of a Trump campaign aide, and has not dissuaded those who say the FBI was determined to block Trump from reaching the White House.

Four days after WikiLeaks published the DNC’s stolen emails, a senior Australian diplomat spoke with a U.S. official “about an ‘urgent matter’ that required an in-person meeting” and subsequently relayed what he had heard about possible Russian assistance from the Trump campaign aide, George Papadopoulos, according to the report.

The information was passed back to Washington, where senior FBI officials discussed it in light of what they believed Russia was already doing. The information was “really disturbing,” one senior FBI official told the inspector general. Andrew McCabe, then the FBI’s deputy director, said it was a “tipping point” in the investigation of Russian interference.

The decision to open Crossfire Hurricane, on July 31, was unanimous, McCabe said.

The FBI team’s first objective was to determine which people associated with Trump’s campaign may have been in a position to receive the alleged Russian assistance, the inspector general found.

In addition to Papadopoulos, the team identified three potential conduits: Carter Page, a campaign adviser; Paul Manafort, the campaign chairman; and Michael Flynn, a senior adviser who would go on to briefly serve as the White House national security adviser. By Aug. 10, the bureau had opened separate counterintelligence investigations on all four men.

Page, an energy consultant who had traveled in Russia, “quickly rose to the top” because he had met previously with known Russian intelligence officers, the inspector general found.

But Page also had a previous relationship with another intelligence organization, the CIA, something the FBI may not have fully understood, according to the report.

On Aug. 17, the CIA advised the FBI in writing that Page had been an “operational contact” from 2008 to 2013, the inspector general found, which meant that the agency could ask Page questions about things he may have learned in the normal course of his activities, but could not assign him to go out and gather information. The report does not identify the agency by name, but it was the CIA, according to people familiar with the matter who spoke on the condition of anonymity to discuss a sensitive issue.

Page, the agency said, had previously provided information about his contacts with Russian intelligence officers. That was potentially an important fact because if Page had been telling the CIA about those Russian contacts, that might make him seem less suspicious to the FBI.

The inspector general found no evidence that the Crossfire Hurricane team asked for more information about Page’s relationship with the CIA before asking a court for a surveillance warrant to monitor him.

After Page publicly stated in May 2017 that he had helped the U.S. government in the past, the FBI renewed efforts to figure out whether Page had been a U.S. intelligence asset, which he was not.

An FBI attorney, who has been identified as Kevin Clinesmith, reached out to a liaison at the CIA for more information. He told investigators for the inspector general and colleagues that he believed he was informed that Page had been some kind of source for the agency, or perhaps had been at meetings at which an agency source was also present, and had been included in the CIA’s reports as a result. In fact, the liaison said those assessments were inaccurate, and information she provided indicated Page had been a direct source of information for the CIA.

But the attorney did not convey that information to colleagues who were concerned about the issue. Instead, Clinesmith forwarded a June 15 email from the liaison to another FBI agent that described Page’s relationship with the CIA, but investigators concluded that he altered the email to add a phrase indicating that Page was “not a source,” changing its meaning.

Clinesmith at first denied altering the email in interviews with investigators but later acknowledged he had done so. Horowitz has referred the issue to prosecutors to investigate whether the alteration amounted to a crime.

The decision to seek a wiretap on Page under the Foreign Intelligence Surveillance Act has been one of the most hotly contested chapters of the Trump-Russia investigation, and the inspector general faulted the bureau’s procedures for “significant inaccuracies and omissions.”

Among those failures, the FBI had gathered information suggesting that reports by former British intelligence officer Christopher Steele, which were a key element justifying the surveillance application, were not as reliable as officials had described to the court that ultimately approved the warrant multiple times.

The number of “basic and fundamental errors” by multiple levels of the bureau “raised significant questions regarding the FBI chain of command’s management and supervision of the FISA process,” the inspector general found. But Horowitz did not conclude the FBI’s application should have been rejected.

The inspector general also scrutinized how the FBI used a confidential source to gather information about Page’s possible Russian connections.

On Aug. 20, Stefan A. Halper, then an emeritus professor at Cambridge, met with Page, whom he had known previously, the inspector general found. Their encounter has also been investigated by Republican lawmakers who suspect the FBI of trying to plant a spy inside the Trump campaign.

The inspector general’s findings appeared to dispel that theory. Halper was not able to draw out any meaningful information about Russia’s intentions. But in August, the FBI team learned that Halper had been offered a job on Trump’s campaign, and agents vociferously objected to using him as a source if he did so, the inspector general found.

“No freaking way,” one case agent told investigators, summarizing the reaction of lawyers advising the team. Crossfire Hurricane “was not ‘pushing for that . . . [because they were] not trying to get into the campaign,’ ” the case agent said.