The F.B.I. Pledged to Keep a Source Anonymous. Trump Allies Aided His Unmasking.

WASHINGTON — Not long after the early 2017 publication of a notorious dossier about President Trump jolted Washington, an expert in Russian politics told the F.B.I. he had been one of its key sources, drawing on his contacts to deliver information that would make up some of the most salacious and unproven assertions in the document.

The F.B.I. had approached the expert, a man named Igor Danchenko, as it vetted the dossier’s claims. He agreed to tell investigators what he knew with an important condition, people familiar with the matter said — that the F.B.I. keep his identity secret so he could protect himself, his sources and his family and friends in Russia.

But his hope of remaining anonymous evaporated last week after Attorney General William P. Barr directed the F.B.I. to declassify a redacted report about its three-day interview of Mr. Danchenko in 2017 and hand it over to Senator Lindsey Graham, Republican of South Carolina and chairman of the Senate Judiciary Committee. Mr. Graham promptly made the interview summary public while calling the entire Russia investigation “corrupt.”

The report blacked out Mr. Danchenko’s name and other identifying information. But within two days, a post on a newly created blog entitled “I Found the Primary Subsource” identified him, citing clues left visible in the F.B.I. document. A pseudonymous Twitter account created in May then promoted the existence of the blog. And the next day, RT, the Kremlin-owned, English-language news and propaganda outlet, published an article amplifying Mr. Danchenko’s identification.