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.
Born in Ukraine, Mr. Danchenko, 42, is a Russian-trained lawyer who earned degrees at the University of Louisville and Georgetown University, according to LinkedIn. He was a senior research analyst from 2005 to 2010 at the Brookings Institution, where he co-wrote a research paper showing that, as a student, President Vladimir V. Putin of Russia appeared to have plagiarized part of his dissertation.