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 opposition research file.
The dossier was reportedly partially funded by Trump-rival Hillary Clinton's campaign and the Democratic party.
What was the testimony about?
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.
What was the Trump blackmail claim?
No evidence has emerged that Mr Trump was blackmailed by the Kremlin.
But in his closed-door session, Mr Simpson told senators: "He [former British spy Christopher Steele] thought from his perspective there was an issue - a security issue about whether a presidential candidate was being blackmailed."
The Fusion GPS founder also said: "Chris said he was very concerned about whether this represented a national security threat and said he wanted to - he said he thought we were obligated to tell someone in government, in our government about this information."
The dossier contained an unsubstantiated claim that Mr Trump was once filmed with prostitutes at a hotel in the Russian capital.
Mr Trump himself poured scorn on that allegation a year ago, saying he has always been on guard against hidden cameras in Moscow hotel suites.
There has been no comment from the FBI about Mr Simpson's testimony.
Who was supposedly killed?
During his 10 hours of congressional testimony, Mr Simpson was cagey on the origins of claims in the dossier.
The Fusion GPS chief's lawyer, Joshua Levy, interrupted to say: "[Mr Simpson] wants to be very careful to protect his sources.
"Somebody's already been killed as a result of the publication of this dossier and no harm should come to anybody related to this honest work."
Mr Levy did not identify the person he said had died.
However, a source told CNN the attorney's remark did not refer to any specific death, but instead alluded to a string of unsolved fatalities of Russians after the 2016 election that have preoccupied conspiracy theorists.
Can Simpson's testimony be trusted?
Several news organisations, including the BBC, were briefed on the dossier before the November 2016 election.
However, most decided not to report on the material because its sometimes lewd content could not be verified.
One of Mr Simpson's previous targets, US-born financier Bill Browder, has called him a liar who acts at Russia's behest.
Mr Browder, a Kremlin critic who was once the subject of damaging allegations by Fusion GPS, told the New York Times about Mr Simpson: "He's a professional smear campaigner and liar for money."
Why was the testimony released now?
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.