Attorney General William Barr said a "problem" in the Justice Department watchdog report on the Russia investigation stems from fired FBI Director James Comey’s refusal to have his security clearance temporarily reinstated.

The former FBI chief, who claimed Inspector General Michael Horowitz's assessment provided him vindication, was brought up by Barr during an interview Tuesday with NBC News' Pete Williams.

Brushing off Horowitz's determination that the FBI's counterintelligence investigation into the Trump campaign was properly predicated, Barr said U.S. Attorney John Durham would need to finish his investigation before any final conclusions are made about the motivations behind the inquiry.

Noting the limits inherent in an inspector general review, including jurisdiction and inability to compel testimony, Barr pointed to Comey as an example where Horowitz's report fell short.

“Durham is not limited to the FBI. He can talk to other agencies. He can compel people to testify,” Barr said. “One of the problems in the IG’s investigation, I think he would agree, is that Comey refused to sign back up for his security clearance and therefore couldn't be questioned about classified matters. So, someone like Durham can compel testimony, he can talk to a whole range of people, private parties, foreign governments, and so forth.”

In his report, Horowitz said Comey was one of a number of former FBI employees who “chose not to request that their security clearances be reinstated for their [inspector general] interviews” and “therefore, we were unable to provide classified information or documents to them during their interviews to develop their testimony, or to assist their recollections of relevant events.”

The focus of Horowitz's investigation was into allegations of Foreign Intelligence Surveillance Act abuse against onetime Trump campaign adviser Carter Page. Horowitz found errors and omissions were made in the process but did not establish that political bias tainted officials' actions throughout the Russia investigation.

Comey was fired by President Trump in May 2017, but there has been some murkiness surrounding Comey's security clearance.

In the summer of 2018, the White House listed Comey among former officials Trump considered relieving of their security clearances. But Comey's friend, Lawfare editor-in-chief Benjamin Wittes, tweeted out that he asked Comey if he still had a clearance to revoke, and the former FBI director said "Nope."

Comey took a victory lap on Twitter following the release of Monday’s report, saying, “So it was all lies. No treason. No spying on the campaign. No tapping Trump's wires. It was just good people trying to protect America.”

But in Barr’s view, Comey's decision, which limited what he could be asked, meant a full account of the FBI’s actions still had not been discovered.

Barr explained his divergence with the DOJ watchdog by pointing out Horowitz, as inspector general, “starts with limited information” and “can only talk to people who are essentially there as employees” and “he's limited to the information generally in the FBI.”

Barr said Horowitz’s approach was “a very deferential standard” of accepting people at their word “as long as there's not contradictory testimonial or documentary evidence,” and so, therefore, in his view, Horowitz hadn’t fully decided the issue of whether there were any improper motivations.

“I think, right now, it would be premature to make any judgment, one way or the other," Barr said.

[ Read more: Barr blasts ‘bogus narrative’ on Trump-Russia controversy hyped by ‘irresponsible press’]

