(Bloomberg) -- Former FBI Director James Comey said the probe into whether people close to Donald Trump conspired with Russia to interfere in the 2016 election was largely conducted “by the book” but that specific aspects of the inquiry fell short.

Pressed on Wednesday by Senate Judiciary Chairman Lindsey Graham, who said the probe was based on faulty assumptions and evidence tampering, Comey said he found the shortcomings -- which included doctoring an email from the CIA -- “deeply disturbing.” But he backed an inspector general’s report that concluded the probe was valid and not driven by bias in the bureau’s leadership.

“This was an investigation that was appropriately predicated and opened, that had to be opened,” Comey said. “And it was in the main conducted in the right way, picked up by the Special Counsel, led to the indictment of dozens of people and a finding by your colleagues in the Senate that the head of Trump’s campaign was a grave counterintelligence threat to the United States of America.”

Graham pushed back, saying the probe relied too much on a discredited dossier funded in part by the Democratic National Committee. Republicans on the panel more broadly focused their questions on problems with the FBI’s request to conduct secret surveillance on a former Trump campaign adviser, Carter Page, and they slammed Comey’s repeated responses that he wasn’t aware of or didn’t recall key details.

“Was this an important case for the FBI or was this kind of a run-of-the-mill thing,” Graham asked. “Every time you found information to put the reliability of the dossier in question, everyone seemed to ignore it and just plowed forward.”

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Barely four weeks from the Nov. 3 election, Senate Republicans are trying to put the spotlight back on what they said was a politically-motivated probe meant to undermine Trump’s candidacy in 2016 and, later, his presidency. Wednesday’s hearing is the third of four scheduled on “Crossfire Hurricane,” with former FBI Deputy Director Andrew McCabe tentatively scheduled for next week.

The debate comes after U.S. intelligence agencies have again assessed that Russia is interfering again in the 2020 presidential race to aid Trump by denigrating his rival, former Vice President Joe Biden.

Graham said he had no doubt that Russia interfered in the 2016 election and is doing so in 2020. But he said he was troubled by evidence that the process to approve secret warrants for national security investigations appears to be broken.

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Comey, who was fired by Trump in May 2017, has been criticized by Republicans and Democrats alike for his actions around the 2016 election, especially his decision to reopen a probe into Hillary Clinton’s emails just days before the election.

It was his first testimony on Capitol Hill since soon after his dismissal more than three years ago.

For Republicans, the hearing serves as a vehicle to claim that Trump was the victim of a witch hunt in the FBI’s Crossfire Hurricane investigation and that Comey, if not politically biased against Trump, was incompetent in his leadership.

“It appears that at the top of your organization there was a culture where many of the people who should have been doing the hard work to make sure the checks and balances were fully carried out didn’t think there was really any chance they would get caught, and so they could be sloppy to malicious,” Nebraska Republican Senator Ben Sasse said.

Republicans also challenged Comey with information recently released by Attorney General William Barr.

For example, Barr told Graham in a Sept. 24 letter that a primary source for a salacious dossier about Trump was the subject of an FBI counterintelligence investigation from 2009 to 2011.

Comey said that wasn’t an issue he recalled being briefed on, and he pushed back on Barr’s repeated criticism of the Russia probe, saying he was “mystified” why the attorney general wouldn’t see how the bulk of the evidence coming before the bureau in 2016 and 2017 wouldn’t merit further examination.

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