U.S. Justice Department Inspector General Michael Horowitz arrives before testifying to the Senate Judiciary Committee in the Hart Senate Office Building on Capitol Hill June 18, 2018 in Washington, DC.

The Department of Justice's internal watchdog on Monday released its long-awaited report on the FBI's decision to investigate Russian interference in the 2016 presidential election, including its surveillance of a then-Trump campaign aide.

Inspector General Michael Horowitz found that there was a proper legal basis for the FBI to seek a surveillance warrant against Carter Page, a former foreign policy advisor to the Trump campaign, according to the report, which runs more than 400 pages in length.

Horowitz also reported that there was no evidence of political bias at the root of the investigation — rebuffing President Donald Trump's repeated claims about a concerted effort to undermine his campaign being led by "deep state" Washington bureaucrats.

"We did not find documentary or testimonial evidence that political bias or improper motivation influenced the FBI's decision" to seek surveillance warrants on Page, Horowitz's report concludes.

Crucially, the report said the so-called Steele dossier — which contained salacious claims about Trump and was held up as a key example of bias in the investigation — "played no role" in the opening of the probe into Trump campaign officials.

But the reporting from British former intelligence officer Christopher Steele did play a "central and essential role" in the decision to apply for a surveillance warrant on Page.

And Horowitz's probe did find "serious performance failures" made by some agents in charge of the surveillance applications submitted to the U.S. Foreign Intelligence Surveillance Court.

Those failures included numerous "factual misstatements omissions" found in the review — some more significant than others, the report said, but when "taken together resulted in [surveillance] applications that made it appear that the information supporting probable cause was stronger than was actually the case."

Despite the indications from early reports that Horowitz's findings would deflate many of Trump's allegations about FBI wrongdoing, the president tweeted Sunday afternoon that the watchdog report "will be the big story!"

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White House press secretary Stephanie Grisham said in a statement Monday afternoon that "The shocking report from the DOJ Inspector General shows an out-of-control FBI under President Obama and former Director Jim Comey."

The evidence in the report "shows a repeated effort to mislead the FISA Court long after the FBI was aware the 'Dossier' was false, phony and could not be used justify spying on the Trump Campaign," Grisham said. "The American people should be outraged and terrified by this abuse of power. This should never happen to another presidential candidate or any American ever again."

Attorney General William Barr, who came under fire in April for asserting that he believed the Trump campaign was spied upon, said in a lengthy statement that the Horowitz report "reflects a clear abuse of the FISA process."

"The Inspector General's report now makes clear that the FBI launched an intrusive investigation of a U.S. presidential campaign on the thinnest of suspicions that, in my view, were insufficient to justify the steps taken," Barr said.

Barr had drawn scrutiny for spearheading a separate, broader investigation into the origins of the Trump-Russia probe by the DOJ itself. He explained in May that he considered the inspector general's powers to be too "limited" to sufficiently answer his questions about the probe into Trump's campaign.

John Durham, the U.S. attorney Barr appointed to that broader probe, said in a statement that "while our investigation is ongoing, last month we advised the Inspector General that we do not agree with some of the report's conclusions as to predication and how the FBI case was opened."

Read the full report from Justice Department Inspector General Michael Horowitz: