The woman had set up the meeting to discuss foreign policy issues. But she was actually a government investigator posing as a research assistant, according to people familiar with the operation. The F.B.I. sent her to London as part of the counterintelligence inquiry opened that summer to better understand the Trump campaign's links to Russia.
The American government's affiliation with the woman, who said her name was Azra Turk, is one previously unreported detail of an operation that has become a political flash point in the face of accusations by President Trump and his allies that American law enforcement and intelligence officials spied on his campaign to undermine his electoral chances. Last year, he called it "Spygate."
The decision to use Ms. Turk in the operation aimed at a presidential campaign official shows the level of alarm inside the F.B.I. during a frantic period when the bureau was trying to determine the scope of Russia's attempts to disrupt the 2016 election, but could also give ammunition to Mr. Trump and his allies for their spying claims.