The FBI lacked “actual evidence” to investigate Donald Trump’s 2016 presidential campaign and relied too heavily on tips provided by Trump’s political opponents to fuel the probe, US Special Counsel John Durham concluded in a report released on Monday.

The report marks the end of a four-year probe launched in May 2019 when then-Attorney General William Barr appointed Durham, a veteran prosecutor, to probe potential missteps by the FBI when it launched its early stage “Crossfire Hurricane” inquiry into potential contacts between Donald Trump’s 2016 presidential campaign and Russia.

That Crossfire Hurricane investigation would later be handed over to Special Counsel Robert Mueller, who in March 2019 concluded there was no evidence of a criminal conspiracy between Trump’s 2016 campaign and Russia.

In his new 306-page report, Durham concluded that US intelligence and law enforcement did not possess any “actual evidence” of collusion between Trump’s campaign and Russia prior to launching Crossfire Hurricane.

He also accused the bureau of treating the 2016 Trump probe differently from other politically sensitive investigations, including several involving Trump’s Democratic rival Hillary Clinton.

For instance, he said Clinton and other officials received defensive briefings about being the possible targets of foreign interference, whereas Trump received no such briefing before the FBI opened probes into four members of his campaign.

“The Department and the FBI failed to uphold their important mission of strict fidelity to the law in connection with certain events and activities described in this report,” Durham wrote.

“Senior FBI personnel displayed a serious lack of analytical rigor towards the information that they received, especially information from politically affiliated persons and entities.”

In response to the report, the FBI said it has already implemented dozens of corrective actions that have been in place for some time.

Durham’s report was released to Congress on Monday without redactions, after it was delivered to Attorney General Merrick Garland on Friday. House Judiciary Committee Republican Chair Jim Jordan said on Twitter he has invited Durham to testify about his report next week.

Durham’s findings are likely to become political fodder for Trump, who is planning to run for re-election in 2024 despite facing criminal charges in New York and two federal investigations by Special Counsel Jack Smith that are looking both at Trump’s retention of classified records and his role in efforts to overturn the results of the 2020 presidential election.

Trump had hoped Durham would release his report ahead of the 2020 election, in what he thought would be a blow to President Joe Biden’s campaign. Trump said on social media on Monday he was pleased with the results of Durham’s report.

But Durham’s investigation has largely failed to produce meaningful impact, after two separate juries acquitted both defendants he tried to prosecute in 2022.

In one case Durham brought, a Washington, D.C. jury acquitted Hillary Clinton’s former campaign attorney Michael Sussmann on charges he lied to the FBI when he met with the bureau in September 2016 to share a tip about possible communications between Trump’s business and a Russian bank.

Durham’s investigation was dealt yet another major set-back just a few months later, when a jury in Virginia acquitted Russian researcher Igor Danchenko of charges that he lied to the FBI when he was interviewed about the sources of information he provided that became part of a document known as the “Steele dossier.”

That document, penned by former British intelligence officer Christopher Steele, made allegations about ties between Trump’s 2016 election campaign and Russia and contained salacious details - many of which were never substantiated.

An investigation by the Justice Department’s inspector general later found that the FBI improperly continued to rely on unsubstantiated allegations in the Steele dossier when it applied for court-approved warrant applications to monitor the communications of Carter Page, a former Trump campaign adviser.

Durham did secure a successful guilty plea against former FBI attorney Kevin Clinesmith, who was singled out in the inspector general’s report, for altering an email that was used to justify a government wiretap application for Page.

Durham’s report on Monday echoed many of the concerns the inspector general previously raised about the rigor of the FBI’s process for applying to the Foreign Intelligence Surveillance Court for wiretap applications.