Steve Bannon believes President Donald Trump may have committed the greatest mistake in modern American political history when he fired FBI Director James Comey this May.

In an interviewed that aired Sunday night on CBS’s 60 Minutes, Bannon — who served as Trump’s chief White House strategist before being pushed out this August — appeared to confirm that he urged against firing Comey. Trump’s sudden dismissal of the FBI director led the Justice Department to appoint Robert Mueller as special counsel in an investigation that could doom the administration.

2. WATCH: Bannon says firing of Comey was the biggest mistake in modern political history. Also, he refuses to answer if Kushner was for it. pic.twitter.com/d0zcZgowd1 — Yashar Ali (@yashar) September 11, 2017

Though Bannon didn’t expound at length of his interpretation of the firing, he suggested that Trump made a mistake by turning the bureau beyond Comey against him.

“I am a big believer, in this city, that this is a city of institutions, not individuals,” he said. “You have to look at it as an institution.”

Here’s Bannon’s exchange with CBS’s Charlie Rose: