In mid-July 2016, a retired American professor approached an adviser to Donald Trump’s presidential campaign at a symposium about the White House race held at a British university.

The professor took the opportunity to strike up a conversation with Carter Page, whom Trump had named a few months earlier as a foreign policy adviser.

But the professor was more than an academic interested in American politics — he was a longtime U.S. intelligence source. And, at some point in 2016, he began working as a secret informant for the FBI as it investigated Russia’s interference in the campaign, according to people familiar with his activities.