Former president Donald Trump, who is likely to use the findings as political fodder, has said the report's findings show "the American Public was scammed".
The FBI lacked any "actual evidence" to begin investigating potential links between Donald Trump's 2016 presidential campaign and Russia, a report has concluded.
US Special Counsel John Durham's probe also found the bureau had relied too heavily on tips provided by the former president's political opponents before it launched its early stage "Crossfire Hurricane" inquiry.
Mr Trump, who will likely use the findings as political fodder as he aims to seek re-election in 2024, wrote on his Truth Social platform: "WOW! After extensive research, Special Counsel John Durham concludes the FBI never should have launched the Trump-Russia Probe! In other words, the American Public was scammed, just as it is being scammed right now by those who don't want to see GREATNESS for AMERICA!"
The report marks the end of a four-year probe launched in May 2019 when then Attorney General William Barr appointed Mr Durham to investigate potential missteps by the FBI when it launched Crossfire Hurricane.
That 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 Mr Trump's campaign and Moscow.
In his new 306-page report, Mr Durham concluded that US intelligence and law enforcement did not possess any "actual
evidence" of collusion between Mr Trump's campaign and Russia prior to launching Crossfire Hurricane.
The report criticised the FBI for opening a full-fledged investigation based on "raw, unanalysed and uncorroborated intelligence," saying the speed at which it did so was a departure from the norm.
It also said investigators repeatedly relied on "confirmation bias", ignoring or rationalising away evidence that undercut their premise of a Trump-Russia conspiracy as they pushed the probe forward.
John Durham's report has criticised the FBI.
He also accused the bureau of treating the 2016 Trump probe differently from other politically sensitive investigations,
including several involving the former president's Democratic rival Hillary Clinton.
For instance, he said Mrs Clinton and other officials received defensive briefings about being the possible targets of foreign interference, whereas Mr Trump received no such briefing before the FBI opened probes into four members of his campaign.
In response to the report, the FBI said it has already implemented dozens of corrective actions that have been in place
for some time.
Mr 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 Mr Durham to testify about his report next week.
The report comes as Mr Trump is planning to run for re-election in 2024 despite facing criminal charges in New York.
Meanwhile, two federal investigations by Special Counsel Jack Smith are looking both at Mr Trump's retention of classified records and his role in efforts to overturn the results of the 2020 presidential election.
Mr Trump had hoped Mr Durham would release his report ahead of the 2020 election, in what he thought would be a blow to President Joe Biden's campaign.