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(CNN) —  

The Justice Department’s internal watchdog sharpened his criticism of the FBI on Wednesday, telling lawmakers he was alarmed that “so many basic and fundamental errors” occurred in the early months of the FBI’s probe into potential Trump-Russia collusion.

“I think the activities we found here don’t vindicate anybody who touched this,” DOJ Inspector General Michael Horowitz told the Senate Judiciary Committee, delivering a stinging rebuke to the former FBI leaders who claimed victory when his sweeping report was released on Monday.

Horowitz also backed up his major conclusion: that the FBI was justified when it launched the Russia probe in July 2016, and senior FBI officials weren’t motivated by political bias against candidate Donald Trump.

In a remarkable display of disunity within the Justice Department, Attorney General William Barr publicly rejected some of these findings and backed up Trump’s insistence that the investigation should have never happened. Asked about the split, Horowitz stood by his conclusions.

Horowitz also uncovered significant problems and mistakes with the surveillance of a former Trump campaign adviser, Carter Page. The court-approved wiretaps were conducted under the Foreign Intelligence Surveillance Act, or FISA, which is typically used for terrorists and spies.

“The FISA process here was not used appropriately, properly, and the rules were not followed,” Horowitz testified. Though he didn’t accuse FBI officials of intentionally abusing the FISA law, he said investigators “certainly misled the court” by omitting exculpatory information from four applications for surveillance. The FISA court ultimately approved all four warrants against Page.

The hearing provided a split-screen moment in Washington. While senators relitigated Russian interference in the 2016 election, members of the House prepared articles of impeachment against Trump, whom Democrats accused of soliciting help from Ukraine for the 2020 election.

RELATED: JJustice Department watchdog investigating possible FBI leaks to Giuliani in 2016

Horowitz versus Barr

The deepening divide within the Justice Department was on full display Wednesday. Over and over, Horowitz staked out positions that were out of step with his bosses – Barr and Trump.

Apparently unsatisfied with the outcome of Horowitz’s review, Barr has pushed forward with a separate investigation into potential misconduct in the Russia investigation. For that probe, Barr tapped US Attorney John Durham, who is now overseeing a criminal investigation, after getting a referral from Horowitz that a low-level FBI lawyer doctored an email during the FISA process.

Both Barr and Durham released statements this week disagreeing with Horowitz’s conclusions. Pressed on Barr’s insistence that the Russia investigation wasn’t justified, Horowitz said, “he’s free to have his opinion. We have our finding, and as I said earlier, I stand by our conclusion.”

Horowitz confirmed reports from CNN and others that his office asked Barr and Durham to share any evidence that could assist his review into the origins of the Russia investigation. He also said he was “surprised” to see Durham issue a public statement undercutting his report.

“None of the discussions changed our findings” that senior FBI officials weren’t motivated by anti-Trump bias when they opened investigations into his campaign aides, Horowitz said.

Later in the hearing, Horowitz rejected the Trump-backed conspiracy theory that President Barack Obama personally ordered the FBI to wiretap Trump and investigate his campaign.

“We certainly didn’t see any evidence of that in the FBI’s files or the (Justice) Department’s files,” Horowitz said, referring to the million-plus documents that his team examined.

No information has emerged to corroborate the claim that Trump’s calls were wiretapped or that Obama was involved. Some top Republicans dismissed the allegations, and the Justice Department said in court filings that it doesn’t have any evidence to back up what Trump said.

California Sen. Dianne Feinstein, the top Democrat on the committee, said the IG report “conclusively refutes” prior claims by Trump and Barr about a biased FBI process.

“This was not a politically motivated investigation. There is no deep state,” Feinstein said. “Simply put, the FBI investigation was motivated by facts, not bias.”

Graham: FBI took law into its own hands

South Carolina Sen. Lindsey Graham, the Republican committee chair, said FBI “abuse” and misconduct reached historic proportions, comparing it to notorious FBI director J. Edgar Hoover.

Going farther than his written report, Horowitz agreed that the FBI’s applications for FISA surveillance were “misleading to the court.” But he refused to speculate whether the errors were so exculpatory that the judges would have rejected the warrants if they had all the information.

“Let me put it this way, I would not have submitted the one they put in,” Horowitz told Graham, referring to the request for FISA surveillance. “…The FISA applications made it appear as though the evidence supporting probable cause was stronger than was actually the case.”

The problems were so systemic, Graham said, that they made him question the legitimacy of the FISA laws, which have become critical tools for US counterterrorism efforts since the 1970s.

“I’m a pretty hawkish guy… I’d hate to lose the ability of the FISA court to operate at a time probably when we need it the most,” Graham said. “But after your report, I have serious concerns about whether the FISA court can continue unless there’s fundamental reform.”

There have been bipartisan attempts to update the FISA laws, and lawmakers from both parties touted their own proposals to reign in the secret FISA court and protect Americans’ privacy. The saga ushered in an unplanned era of transparency: The Justice Department was forced to declassify and release the Page FISAs after a lawsuit last year, the first time in US history.

A spokesperson for the FISA court declined to comment after the Horowitz report came out.

“No evidence” to support Steele dossier claims

When the FBI asked the FISA court to wiretap Page, they said they wanted to investigate whether he was acting as agent of the Russian government, based in part on allegations from “the dossier,” a series of memos compiled by former British spy Christopher Steele.

Steele’s research was indirectly funded by the Democratic National Committee and Hillary Clinton’s campaign. Trump has maintained that all the information in the dossier is “fake,” even though Steele’s big-picture reporting that Russia was attacking the US election proved true.

The dossier accused Page of acting as a conduit between the Trump campaign and the Russian government. Steele claimed that during Page’s highly publicized July 2016 trip to Moscow, he met a top Russian energy executive and a Kremlin official to facilitate the alleged scheme, which included offers of lucrative energy deals in exchange for Trump lifting sanctions.

The FISA court approved surveillance and wiretapping for nearly a year, approving warrants sought by Obama and Trump administration officials. But in the end, Horowitz said the FBI didn’t find any evidence to corroborate the allegations about Page that were mentioned in the dossier.

“The FBI, at the end of these FISAs, told us that they had found no evidence to corroborate the allegations in the Steele dossier related to him,” Horowitz told lawmakers.

The sweeping investigation led by special counsel Robert Mueller confirmed that Russia meddled in the 2016 election but did not establish that there was a criminal conspiracy of collusion between Trump’s team and the Russians. Horowitz said he backed Mueller’s findings.

“We don’t take issue with any part of the special counsel’s report,” Horowitz said.

Mueller didn’t bring charges against Page, who maintains that he never did anything wrong and was improperly snooped on by the US government. Horowitz’s comments served as vindication for Page.

Horowitz reviewed the early stages of the Russia investigation, which was conducted by the FBI, until the probe was handed off to Mueller after he became the special counsel in May 2017.

CNN’s Katelyn Polantz contributed to this report.