The ex-British spy who compiled a dossier on Donald Trump feared he was being blackmailed by the Kremlin, US Congress has heard.
Former intelligence agent Christopher Steele took his concerns to the FBI in July 2016, the Senate was told.
It was also claimed during the hearing that someone has been killed because of the dossier.
The opposition research was reportedly funded by Trump-rival Hillary Clinton's campaign and the Democratic party.
Tuesday's allegations emerged in a 312-page transcript of a Senate Judiciary Committee hearing from last August.
It featured testimony by Glenn Simpson, head of Fusion GPS, the Washington firm that commissioned the Steele dossier.
Mr Simpson defended the research file, which purported to show financial and personal links between Mr Trump, his advisers and Moscow.
The Senate committee is one of three congressional panels investigating Russia's alleged meddling in the 2016 US presidential election and claims of collusion with the Trump campaign.
The White House and many Republicans have argued the Fusion GPS dossier and claims of collusion are a political smear, devoid of credibility.
Senator Dianne Feinstein, a California Democrat, released the transcript of the hearing from five months ago, saying Mr Simpson had asked her to do so.
She said she had taken the step because of the "innuendo and misinformation" circulating about Fusion GPS.
Ms Feinstein said this was "part of a deeply troubling effort to undermine the investigation into potential collusion and obstruction of justice".
It comes after the committee's Republican chairman, Charles Grassley, last week called for a criminal investigation into Mr Steele, reports BBC.