The United States Justice Department’s internal watchdog will release a highly anticipated report on Monday that is expected to reject President Donald Trump‘s claims that the Russia investigation was illegitimate and tainted by political bias from FBI leaders. But it is also expected to document errors during the investigation that may animate Trump supporters.

The report, as described by people familiar with its findings, is expected to conclude there was an adequate basis for opening one of the most politically sensitive investigations in FBI history and one that Trump has denounced as a “witch-hunt”. It began in secret during Trump’s 2016 presidential run and was ultimately taken over by then-Special Counsel Robert Mueller.

The report comes as Trump faces an impeachment inquiry in Congress centred on his efforts to press Ukraine to investigate a political rival, Democrat Joe Biden – a probe the president also claims is politically biased.

Still, the release of Inspector General Michael Horowitz’s review is unlikely to quell the partisan battles that have surrounded the Russia investigation for years. It is also not the last word: A separate internal investigation continues, overseen by Trump’s Attorney General William Barr and led by US Attorney John Durham. That investigation is criminal in nature, and Republicans may look to it to uncover wrongdoing that the inspector general was not examining.

Trump tweeted on Sunday: “I.G. report out tomorrow. That will be the big story!”

Horowitz report vs Durham report

Trump has previously said that he was awaiting Horowitz’s report but that Durham’s report might be even more important.

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Horowitz’s report is expected to identify errors and misjudgements by some law enforcement officials, including by an FBI lawyer suspected of altering a document related to the surveillance of a former Trump campaign aide. Those findings will probably fuel arguments by Trump and his supporters that the investigation was flawed from the start.

But the report will not endorse some of the president’s theories on the investigation, including that it was a baseless “witch-hunt” or that he was targeted by an Obama administration Justice Department desperate to see Republican Trump lose to Democrat Hillary Clinton in 2016.

It is also not expected to undo Mueller’s findings or call into question his conclusion that Russia interfered in that election in order to benefit the Trump campaign and that Russians had repeated contacts with Trump associates.

Some of the findings were described to The Associated Press news agency on condition of anonymity by people who were not authorised to discuss a draft of the report before its release. The AP has not viewed a copy of the document.

It is unclear how Barr, a strong defender of Trump, will respond to Horowitz’s findings. He has told Congress that he believed “spying” on the Trump campaign did occur and has raised public questions about whether the counterintelligence investigation was done correctly.

Steele dossier

The FBI opened its investigation in July 2016 after receiving information from an Australian diplomat that a Trump campaign adviser, George Papadopoulos, had been told before it was publicly known that Russia had dirt on the Clinton campaign in the form of thousands of stolen emails.

By that point, the Democratic National Committee (DNC) had been hacked, an act that a private security firm – and ultimately US intelligence agencies – attributed to Russia. Prosecutors allege that Papadopoulos learned about the stolen emails from a Maltese professor named Joseph Mifsud. Papadopoulos pleaded guilty to lying to the FBI about that interaction.

The investigation was taken over in May 2017 by Mueller, who charged six Trump associates with various crimes as well as 25 Russians accused of interfering in the election either through hacking or a social media disinformation campaign. Mueller did not find sufficient evidence to charge a criminal conspiracy between the Trump campaign and Russia.

He examined multiple episodes in which Trump sought to seize control of the investigation, including by firing James Comey as FBI director, but declined to decide on whether Trump had illegally obstructed justice.

The inspector general’s investigation began in early 2018. It focuses in part on the FBI’s surveillance of a former Trump campaign adviser, Carter Page. The FBI applied in late 2016 for a warrant from the secretive Foreign Intelligence Surveillance Court to monitor Page’s communications, with officials expressing concern that he might have been targeted for recruitment by the Russian government.

Page was never charged and has denied any wrongdoing.

The FBI opened its investigation in July 2016 [File: Danny Moloshok/Reuters]

Republican Senator Lindsey Graham, the chairman of the Senate Judiciary Committee, which is scheduled to hear testimony from Horowitz on Wednesday, said he expected the report would be “damning” about the process of obtaining the warrant.

“I’m looking for evidence of whether or not they manipulated the facts to get the warrant,” Graham said on Fox News Channel’s Sunday Morning Futures programme.

The warrant was renewed several times, including during the Trump administration. Republicans have attacked the procedures because the application relied in part on information gathered by an ex-British intelligence operative, Christopher Steele, whose opposition research into the Trump campaign’s connections to Russia was funded by Democrats and the Clinton campaign.

In pursuing the warrant, the Justice Department referred to Steele as “reliable” from previous dealings with him. Though officials told the court that they suspected the research was aimed at discrediting the Trump campaign, they did not reveal that the work had been paid for by Democrats, according to documents released last year.

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Steele’s research was compiled into a dossier that was provided to the FBI after it had already opened its investigation.

The report also examined the interactions that senior Justice Department lawyer Bruce Ohr had with Steele, whom he had met years earlier through a shared professional interest in countering Russian organised crime. Ohr passed along to the FBI information that he had received from Steele but did not alert his Justice Department bosses to those conversations.

Ohr has since been a regular target of Trump’s ire, in part because his wife worked as a contractor for Fusion GPS, the political research firm that hired Steele for the investigation.

This is the latest in a series of reports that Horowitz, a former federal prosecutor and Obama appointee to the watchdog role, has released on FBI actions in politically charged investigations.

Last year, he criticised Comey for a news conference announcing the conclusion of the Clinton email investigation, and for then alerting Congress months later that the probe had been effectively reopened. In that report, too, Horowitz did not find that Comey’s actions had been guided by partisan bias.