"In the main, it was done by the book, it was appropriate and it was essential that it be done," Comey said, while acknowledging there "were parts of it that were concerning."
Since his ouster, Comey has consistently been one of the Trump's favorite political targets. The president has casted Comey as a villain who orchestrated a 'deep-state' effort to thwart his presidency.
The hearing was the latest in a series organized by Senate Judiciary Chairman Lindsey Graham scrutinizing the activities of investigators involved in "Crossfire Hurricane." Democrats have accused Graham of using the committee as a political tool in the months leading up to the election to bolster the president's claims that he was unfairly targeted by Obama administration officials who investigated his campaign.
Graham has defended his inquiry as born out of a separate review of the Russia investigation by Justice Department Inspector General Michael Horowitz, who uncovered a series of errors on the part of FBI agents involved in the surveillance of former Trump campaign aide Carter Page while determining the Russia investigation as a whole was launched with proper cause.
Among other issues highlighted by Graham, Horowitz criticized the FBI for its reliance in the Page surveillance on a dossier compiled by former British spy Christopher Steele which included salacious allegations that Trump had been compromised by Russia. Attorney General William Barr recently declassified information for Graham's committee revealing one of Steele's primary sources for the dossier had previously been investigated for his ties to Russian intelligence.
The FBI, however, did not rely principally on the Steele dossier in its broader investigation of the Trump campaign's ties to Russia, Sen. Dianne Feinstein noted during the hearing.
Comey largely distanced himself from the surveillance process during the hearing, saying he first learned of the Steele dossier "sometime towards the end of September 2016."
"How could the FBI Director not know about all of this?" Graham pressed Comey during an exchange.
"I can only speculate because it didn't," Comey said. "And as I said the investigation overall was incredibly important. The piece you're focusing on is obviously important but a much smaller slice of all this."
The hearing also followed an extraordinary disclosure Tuesday by Director of National Intelligence John Ratcliffe that he declassified, for Graham's committee, an unverified and potentially fabricated analysis from Russian intelligence in 2016 that Hillary Clinton was working to tie Trump to Russia to distract from the controversy over her private email server.
The intelligence had reportedly been dismissed previously by a separate Senate investigation and was described as "Russian disinformation" by the ranking member of the Senate Intelligence Committee. Ratcliffe in a statement later refuted it was Russian disinformation and said he would brief members of Congress soon on the nature of the intelligence.
Asked by Graham about the intelligence report, Comey said he did not remember ever seeing such an investigative referral during his time as FBI director.
"That doesn't ring a bell with me," Comey said.
Comey was also asked about a key episode in the FBI's investigation into former national security adviser Michael Flynn.
Flynn’s lawyers and the Justice Department, in their efforts to dismiss the case against Flynn, have cited a Jan. 5, 2017 Oval Office meeting in which Comey first briefed President Barack Obama on the FBI’s investigation into Flynn’s contacts with the former Russian ambassador, which Flynn later lied about in an interview with the FBI.
Notes taken by former FBI lawyer Peter Strzok state that Comey described to Obama that intercepts of Flynn’s calls with the ambassador were "legit," which Flynn’s attorneys have clung to in claiming that meant Comey believed the calls were appropriate.
But Comey contradicted that argument in his testimony, stating that if he described them as "legit" at the time it meant he was referring to them as "authentic" and not fabricated.
Under questioning from Democrats, Comey agreed with their criticism of Trump and concerns that he might work to solicit foreign help in the current presidential election against former Vice President Joe Biden.
He also stated that Obama and Biden "never" asked him to investigate a political rival, which Trump has done repeatedly in public demands to the Justice Department over Twitter and in speeches.
"It would compromise the independence of the Justice Department and the FBI's work," Comey said. "It would introduce politics into what should be a fact-driven process."
Comey also took aim at Attorney General Barr for Barr’s comments that the FBI’s investigation of the Trump campaign was "completely baseless" and should never have happened.
"He says that a lot I have no idea what on earth he’s talking about," Comey said. "This was an investigation that was appropriately predicated and that had to be opened 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' because he was funneling information to a known Russian intelligence officer."
Comey added, "The notion that the attorney general believes that was an illegitimate endeavor to investigate mystifies me."
Comey also said that the president’s behavior since taking office has led him to privately speculate that the Russians might still have some sort of compromising information on Trump.
Pressed by Republican Sen. Mike Lee, who dismissed Comey’s speculation as absurd, Comey said he believed it, "because I have eyes and ears," and pointed to instances like Trump’s press conference with Russian President Vladimir Putin in Helsinki when he suggested he agreed with Putin’s denial of election meddling over the assessment by America’s intelligence agencies.