Ms. Turk went to London to help oversee the politically sensitive operation, working alongside a longtime informant, the Cambridge professor Stefan A. Halper. The move was a sign that the bureau wanted in place a trained investigator for a layer of oversight, as well as someone who could gather information for or serve as a credible witness in any potential prosecution that emerged from the case.

A spokesman for the F.B.I. declined to comment, as did a lawyer for Mr. Halper, Robert D. Luskin. Last year, Bill Priestap, then the bureau’s top counterintelligence agent who was deeply involved in the Russia inquiry, told Congress during a closed-door hearing that there was no F.B.I. conspiracy against Mr. Trump or his campaign.

The London operation yielded no fruitful information, but F.B.I. officials have called the bureau’s activities in the months before the election both legal and carefully considered under extraordinary circumstances. They are now under scrutiny as part of an investigation by Michael E. Horowitz, the Justice Department inspector general. He could make the results public in May or June, Attorney General William P. Barr has said. Some of the findings are likely to be classified.

It is unclear whether Mr. Horowitz will find fault with the F.B.I.’s decision to have Ms. Turk, whose real name is not publicly known, meet with Mr. Papadopoulos. Mr. Horowitz has focused among other things on the activities of Mr. Halper, who accompanied Ms. Turk in one of her meetings with Mr. Papadopoulos and also met with him and other campaign aides separately. The bureau might also have seen Ms. Turk’s role as essential for protecting Mr. Halper’s identity as an informant if prosecutors ever needed court testimony about their activities.

Mr. Barr reignited the controversy last month when he told Congress, “I think spying on a political campaign is a big deal.” He backed off the charged declaration later in the same hearing, saying: “I think spying did occur. The question is whether it was adequately predicated. And I’m not suggesting that it wasn’t adequately predicated. But I need to explore that.”