The conversation at a London bar in September 2016 took a strange turn when the woman sitting across from George Papadopoulos, a Trump campaign adviser, asked a direct question: Was the Trump campaign working with Russia?

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 to as part of the counterintelligence inquiry opened that summer to better understand the Trump campaign’s links to Russia…

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.