After opening the Russia investigation about a month later, they took steps to monitor the details of the investigation more closely than in a typical national security investigation, including using the informant to find out information from the unsuspecting targets. Sending F.B.I. Agents to interview them would have created additional risk that the investigation's existence would come to the fore in the last weeks of a heated presidential race.
F.B.I. The officers concluded that they had the legal authority to initiate the investigation after they learned that Moscow had compromised information about Ms. Clinton in the form of "thousands of e-mails" months before WikiLeaks stole news from Democratic officials released. As part of the operation, codenamed Crossfire Hurricane, the F.B.I. It also began investigating Trump's pollster Paul Manafort and his future National Security Advisor Michael T. Flynn.
Informants take great risks when working for intelligence agencies, Christopher A. Wray, F.B.I. Director, testified before the congress on Wednesday. Their identity should not be compromised, he said, pointing to the efforts of Congress to preserve the name of the source. "The day we can not protect human sources is the day when the American people become less secure."
One of Mr. Trump's attorneys, Rudolph W. Giuliani, confirmed on Friday that neither the President nor his legal department knew for sure that the FBI had implanted a spy in the Trump campaign, as he and the president had alleged.
"I'm not sure, and the president, if there really was one," Mr Giuliani said on CNN. "For a long time, we were told that there was a kind of infiltration."
The informant is well-known in Washington circles after serving in former Republican governments and as a source of information for the C.I.A. in recent years, according to a person who is familiar with the work of the source.