In the final months of Mr. Trump’s presidential campaign, Russian agents escalated a yearlong effort to hack and harass his Democratic opponents, culminating in the release of thousands of emails stolen from prominent Democrats and party officials.
Facebook had said nothing publicly about any problems on its own platform. But in the spring of 2016, a company expert on Russian cyberwarfare spotted something worrisome. He reached out to his boss, Mr. Stamos.
Mr. Stamos’s team discovered that Russian hackers appeared to be probing Facebook accounts for people connected to the presidential campaigns, said two employees. Months later, as Mr. Trump battled Hillary Clinton in the general election, the team also found Facebook accounts linked to Russian hackers who were messaging journalists to share information from the stolen emails.
Mr. Stamos, 39, told Colin Stretch, Facebook’s general counsel, about the findings, said two people involved in the conversations. At the time, Facebook had no policy on disinformation or any resources dedicated to searching for it.
Mr. Stamos, acting on his own, then directed a team to scrutinize the extent of Russian activity on Facebook. In December 2016, after Mr. Zuckerberg publicly scoffed at the idea that fake news on Facebook had helped elect Mr. Trump, Mr. Stamos — alarmed that the company’s chief executive seemed unaware of his team’s findings — met with Mr. Zuckerberg, Ms. Sandberg and other top Facebook leaders.
Ms. Sandberg was angry. Looking into the Russian activity without approval, she said, had left the company exposed legally. Other executives asked Mr. Stamos why they had not been told sooner.
Still, Ms. Sandberg and Mr. Zuckerberg decided to expand on Mr. Stamos’s work, creating a group called Project P, for “propaganda,” to study false news on the site, according to people involved in the discussions. By January 2017, the group knew that Mr. Stamos’s original team had only scratched the surface of Russian activity on Facebook, and pressed to issue a public paper about their findings.