Attorney General William Barr on Monday rejected a key conclusion of an investigation conducted by his own agency's watchdog that a probe into Russian interference into the 2016 election was justified.

Barr, in a lengthy statement, called the FBI's investigation into Moscow's interference "intrusive" and said it had been launched "on the thinnest of suspicions" — even though the Justice Department's inspector general report released Monday concluded that the overall probe was justified and not motivated by politics.

"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.

He added that "the evidence produced by the investigation was consistently exculpatory."

The report by Justice Department Inspector General Michael Horowitz concluded that the FBI and the Justice Department launched their investigation into the 2016 campaign not for political reasons, but because of evidence the Russian government was using cutouts to reach out to the Trump campaign as part of its efforts to influence the election.

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The report found the FBI mishandled parts of its application to monitor a Trump campaign aide as it was probing possible Russian interference in the 2016 race, but that the overall probe was justified. That last conclusion rebuts President Donald Trump's claims that the probe was launched as part of a politically biased plot against him.

At the same time, the IG report found enough errors — and in at least one case, alleged document tampering by a low-level FBI lawyer — with the FBI's applications to a national security court to conduct secret surveillance on a Trump aide that the inspector general is launching a separate inquiry into how the FBI obtains national security warrants to eavesdrop on American citizens.

Barr seized on that finding in his statement, saying that the FBI "investigation and surveillance was pushed forward for the duration of the campaign and deep into President Trump’s administration."

"In the rush to obtain and maintain FISA surveillance of Trump campaign associates, FBI officials misled the FISA court, omitted critical exculpatory facts from their filings, and suppressed or ignored information negating the reliability of their principal source," Barr said, adding that "the malfeasance and misfeasance detailed in the Inspector General's report reflects a clear abuse of the FISA process."

Barr went on to say that the Justice Department "follow all appropriate processes and procedures" recommended by the report, "including as to any potential disciplinary action." He added that he had "full confidence" in FBI Director Chris Wray.

In addition, John Durham, U.S. attorney for Connecticut — whom Barr tapped in May to conduct a special investigation into the larger question of whether any of the U.S. government efforts to investigate Russian election interference involved improper surveillance of the Trump campaign — also criticized the IG report.

"Based on the evidence collected to date, and 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," Durham said in a statement.

Durham's investigation has become a criminal probe, people familiar with the matter have said, though it is unclear what possible crimes are being examined.

Wray, reacting to the release of the IG report in an interview with ABC News, said that one key takeaway for him was that "the Inspector General did not find political bias or improper motivations impacting the opening of the investigation or the decision to use certain investigative tools during the investigations."

"Including FISA?" ABC's Chief Justice Correspondent Pierre Thomas asked.

"Including FISA," Wray said.

During a news conference Monday afternoon, Sen. Lindsey Graham, R-S.C., said that even if the Russia probe was launched properly, "it sure as hell didn’t end okay."

Graham, the chairman of the Senate Judiciary Committee, said that he believes there will be no debate among lawyers "about how the system not only get off the rails, but in my view, became a criminal enterprise to defraud the FISA court, to deny American citizen Carter Page his constitutional rights and to continue an operation against President Trump as president of United States that I think was fundamentally flawed and unlawful."

Graham added, “If I were Carter Page, I’d hire a lawyer, and sue the hell out of the United States.”