(WASHINGTON) — Former FBI Director James Comey in a Senate hearing Wednesday sought to defend the bureau’s efforts to investigate President Donald Trump and his 2016 campaign’s ties to Russia, amid attacks from Republican senators looking to tie him directly to some of the investigation’s missteps.

“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.”

Comey approved the launch of the FBI’s investigation into Russian interference in the 2016 election and ties between Russia and members of Trump’s campaign, dubbed ‘Crossfire Hurricane.’ Trump fired Comey in May 2017 citing the FBI’s Russia probe, prompting the appointment of Robert Mueller as special counsel.

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.’

Graham’s inquiry was 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.

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