The United States Justice Department’s internal watchdog said it found numerous errors but no evidence of political bias by the FBI when it opened an investigation into contacts between Donald Trump‘s presidential campaign and Russia in 2016.

The findings undercut Trump’s claim that he was the target of a “witch-hunt”.

The report by Inspector General Michael Horowitz was likely to give ammunition to both Trump’s supporters and his Democratic critics in the continuing debate about the legitimacy of an investigation that shadowed the first two years of his presidency.

The report’s nuanced conclusions, however, deny a clear-cut vindication for Trump’s supporters or critics. It rejects theories and criticism spread by Trump and his supporters while also finding errors and misjudgments likely to be exploited by Republican allies as the president faces a probable impeachment vote this month.

The president has also sought to downplay expectations, saying repeatedly that he was more eager for the report of John Durham, the hand-picked prosecutor selected by Attorney General William Barr to conduct a separate review of the Russia probe.

Barr rejected the inspector general’s conclusion that there was sufficient evidence to open the investigation.

“The Inspector General’s report now makes clear that the FBI launched an intrusive investigation of a US presidential campaign on the thinnest of suspicions that, in my view, were insufficient to justify the steps taken,” Barr said in a statement.

‘Significant inaccuracies or omissions’

In its 434-page report that examined one million documents, the inspector general said the FBI had an “authorised purpose” when it began its investigation, which was called Crossfire Hurricane at the time, in July 2016.

But the report identified 17 “significant inaccuracies or omissions” in applications for a warrant from the secretive Foreign Intelligence Surveillance Court to monitor the communications of former Trump campaign adviser Carter Page and subsequent warrant renewals. The errors, the watchdog said, resulted in “applications that made it appear that the information supporting probable cause was stronger than was actually the case.”

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When it asked for court permission to open a wiretap on Page, Horowitz found that the FBI relied heavily on research assembled by former British intelligence officer Christopher Steele.

The watchdog found that the FBI had overstated the significance of Steele’s past work as an informant, omitted information about one of Steele’s sources who Steele had called a “boaster” and who Steele said the source “may engage in some embellishment”.

But the report also found the bureau was justified in eavesdropping on Page and that there was not documented or testimonial evidence of any political bias.

The report’s release, coming the same day as a House Judiciary Committee impeachment hearing centred on the president’s interactions with Ukraine, brought fresh attention to the legal and political investigations that have entangled the White House from the moment Trump took office.

The investigation, which was ultimately taken over by then-special counsel Robert Mueller, began after the FBI learned that a former Trump campaign aide, George Papadopoulos, had been saying before it was publicly known that Russia had dirt on Democratic opponent Hillary Clinton in the form of stolen emails.

Those emails, which were hacked from Democratic email accounts by Russian intelligence operatives, were released by WikiLeaks in the weeks before the election in what US officials have said was an effort to harm Clinton’s campaign and help Trump.

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Months later starting the investigation in 2016, the FBI sought and received the Page warrant. Officials were concerned that Page was being targeted for recruitment by the Russian government, though he has denied wrongdoing and has never been charged with a crime.

Republicans have long criticised the process since the FBI relied in part on opposition research from Steele, whose work was financed by Democrats and the Clinton campaign, and that fact was not disclosed to the judges who approved the warrant.

The release of 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 Barr and led Durham. That investigation is criminal in nature, and Republicans may look to it to uncover wrongdoing that the inspector general was not examining.