Facebook CEO Mark Zuckerberg claimed Thursday that he had not known his company had hired a Republican opposition-research firm to attack Facebook’s critics.

A bombshell New York Times report published Wednesday revealed that Facebook had hired Definers Public Affairs, a Republican opposition firm, to attack Facebook critics amid a firestorm over how it handled Russian disinformation efforts on Facebook in the 2016 election. Definers’ tactics included posting stories on a website connected to the company that posed as a news outlet and pressing reporters to investigate whether billionaire financier George Soros was behind attacks on the social giant, according to the Times.

“I learned about this relationship when I read the New York Times piece,” Zuckerberg said. “As soon as I read about this from the New York Times, I got on the phone with our team and we’re no longer working with this firm.”

Facebook fired Definers later on Wednesday, after Definers’ attempt to get reporters to look into whether Soros was behind an anti-Facebook group were criticized as an echo of anti-Semitic tropes about wealthy Jews being puppet masters.

But Definers’ contract with Facebook was far from a secret. A number of reporters had been contacted by Definers on Facebook’s behalf, according to the Times, and Facebook claims the relationship was “well known” in a press release early Thursday.

Zuckerberg insisted he hadn’t known about the Definers contract, although he declined to say who in Facebook had set it up. Zuckerberg said Facebook needed to look into whether it had contracts with any other groups like Definers and consider cancelling them as well.

"This type of firm might be normal in Washington, but it's not the type of thing that I want Facebook to be associated with,” he said.

Zuckerberg also claimed that he didn’t know about Definers’ attacks on Soros, a frequent critic of Facebook.

“I have tremendous respect for George Soros, even though we disagree on the impact of the internet,” Zuckerberg said.