In the days immediately after the 2016 election, Facebook C.E.O. Mark Zuckerberg seemed offended by suggestions that the social network he created might have had any influence on the outcome, beyond serving as a marketplace for the exchange of ideas. “Personally, I think the idea that fake news on Facebook, it’s a very small amount of the content, influenced the election in any way is a pretty crazy idea,” he said, on stage at the Techonomy conference in Half Moon Bay, California. While some were quick to blame Facebook for amplifying misinformation about Hillary Clinton, Zuckerberg suggested that critics were betraying a “profound lack of empathy” by not taking voters who supported Donald Trump seriously.

Zuckerberg wasn’t wrong to be skeptical of Democrats assigning blame rather than engaging in self-reflection. But in dismissing the possibility that social media might be anything other than a force for good, Zuckerberg was also slow to recognize Facebook’s own vulnerabilities in an age of information warfare. About a week after the Techonomy conference, however, Zuckerberg received a “wake-up call” from President Barack Obama, The Washington Post reports. During a meeting of world leaders in Lima, Peru, nine days after the election, Obama tried to personally appeal to Zuckerberg, warning that unless Facebook did something, its fake-news problem would only be exacerbated in the next presidential election:

For months leading up to the vote, Obama and his top aides quietly agonized over how to respond to Russia’s brazen intervention on behalf of the Donald Trump campaign without making matters worse. Weeks after Trump’s surprise victory, some of Obama’s aides looked back with regret and wished they had done more.

Zuckerberg acknowledged the problem posed by fake news. But he told Obama that those messages weren’t widespread on Facebook and that there was no easy remedy, according to people briefed on the exchange, who spoke on the condition of anonymity to share details of a private conversation.

The account of Zuckerberg’s post-election reckoning also reveals new details about what Facebook executives knew, and when they knew it. The Post reports that Facebook notified the F.B.I. as early as June 2016 when a hacking group working in connection to the GRU, the Russian military intelligence unit, started making fake Facebook profiles to disseminate stolen e-mails and manipulate public opinion—days before Guccifer 2.0, a hacking persona now thought to be a front for Russian intelligence, took credit for hacking the Democratic National Committee. But after looking into the accounts, which were linked to the GRU’s hacking group called APT28 or Fancy Bear, which set up a Facebook profile for Guccifer 2.0 and a Facebook page called DCLeaks, the company came to believe they weren’t linked to a foreign government but were instead financially motivated.

It took months for Facebook to publicly acknowledge what was clear to the Obama administration much earlier. Now, nearly a year after the election, Zuckerberg has backtracked on his initial dismissals about fake news. Last week, amid bipartisan congressional furor over what some lawmakers said was Facebook’s unwillingness to share information about Russian interference, Zuckerberg announced online that Facebook would be handing congressional investigators more than 3,000 ads it believes were purchased by a Russian troll farm from 2015 to 2017; he conceded that Facebook had been manipulated during the election; and outlined nine steps he says Facebook will be taking to ensure election interference can’t happen on its platform in the future.