Former US President Barack Obama warned Facebook CEO Mark Zuckerberg about the impact of fake news on the 2016 US presidential election nine days after the chief executive dismissed the idea as “crazy,” according to a Washington Post report published Sunday.

The two met in private at a global forum for world leaders held in Lima, Peru on November 19, according to people briefed on the exchange.

Obama told Zuckerberg that if Facebook did not do more to address the threat, the problem would only continue to get worse in elections to come. Zuckerberg in turn assured Obama that he was aware of the problem, but such messages were not widespread on Facebook and there was no easy solution.

In a year busy with elections around the world, Facebook has increasingly put in place solutions to counter fake news, which seem to have taken effect. The fake news that plagued 2016 US election and the 2017 UK election did not seem to play as significant a role in the German election this month, for example.

But Facebook has still been slow in acknowledging its role in allowing fake news to impact the outcome of last year’s presidential race. Only this month did the social network disclose that it sold $100,000 worth of ads to inauthentic accounts linked to Russia during the election. Last week it said it would hand over 3,000 Russia-linked ads to the Senate and House intelligence committees.

Zuckerberg promised to keep working on solutions, but emphasized Facebook could not prevent all wrongdoing on the social network. “There will always be bad people in the world, and we can’t prevent all governments from all interference,” Zuckerberg said. “But we can make it harder. We can make it a lot harder.”

Representatives for Facebook and Barack Obama didn’t immediately respond to request for comment.