John Podesta, the former failed campaign manager for Hillary Clinton’s presidential bid, urged social media companies to police freedom of speech on their platforms.

“I think that again the social media companies have a responsibility to try to do a better job of knocking off fake sites and be able to police to some extent the propagation of fake news,” he said during a conversation on a podcast operated by former Obama aides.

Podesta said that the propagation of “fake news” was a national security threat, complaining that “Russian bots and trolls” helped spread negative stories about Hillary Clinton across the Internet.

He complained that “alt-right sites” on the internet were “peddling” the worst kind of fake news online.

But he admitted that it might be impossible to fully police content on social media, a “problem” that would have to fought within the American democracy.

“You can’t just ignore it … you have to deal with it and come to terms with it,” he said, offering advice for future Democratic candidates.

He admitted that Hillary Clinton’s campaign failed to use social media as effectively as President Barack Obama did in his campaigns. He added that those channels were “flooded” with information from “dubious sources” complaining that his campaign didn’t do enough to fight back.

“I think in our campaign [we] didn’t do enough persuasion through social media channels, I think we relied overwhelmingly on television for persuasion and used social media largely to talk to our own supporters and activists.”