Ex-British intelligence officer Christopher Steele, who compiled the dossier alleging connections between the Kremlin and then-candidate Donald Trump, alerted the FBI last year out of fear that the allegations left Mr. Trump vulnerable to blackmail, according to the Senate testimony of the opposition researcher who hired him.
Josh Levy, the lawyer who accompanied Glenn Simpson, the onetime Wall Street Journal reporter who helped found the private intelligence service Fusion GPS, also told congressional investigators during the interview session that "somebody's already been killed" because of the unauthorized release of Mr. Steele's unverified findings shortly after Mr. Trump's unexpected electoral victory in November 2016.
On Tuesday, over vocal objections from her Republican colleagues, California Sen. Dianne Feinstein, the ranking Democrat on the Senate Intelligence Committee, released the 312-page transcript of its closed-door interview with Mr. Simpson in August, a unilateral decision Republican committee Chairman Charles E. Grassley of Iowa called "confounding" and damaging to the panel's still-ongoing probe.
Mr. Steele "said he was professionally obligated" to go to the FBI, Mr. Simpson told the committee. "He thought from his perspective there was an issue - a security issue about whether a presidential candidate was being blackmailed."
Mr. Simpson also maintained the FBI told Mr. Steele they had already received information of Trump-Russia ties, including from a source within the Trump campaign.
The transcript covers a range of the most sensitive issues at the heart of the Russian election-meddling saga, including Mr. Trump's tax history, alleged connections to Russian organized crime figures and Fusion GPS' interaction with the FBI. It also addresses material Mr. Steele feared could lead to Kremlin blackmail - allegations that Russian agents recorded Mr. Trump engaging in "perverted sexual acts" in a Moscow hotel suite.
Doubts about Mr. Simpson's credibility, however, have swirled for months and intensified in December when reports, confirmed by court documents and bank records, revealed that his firm hired the wife of a senior Justice Department official to assist in the Trump investigation. Last month, Nellie Ohr's husband, Bruce Ohr, was demoted at the Justice Department.
Republicans claimed the Ohr-Fusion GPS relationship underscored the department's political bias, helping fuel special counsel Robert Mueller's investigation into alleged collusion between the Trump campaign and Russia.
Late last year, new details also emerged about the dossier's funding. Initially, it was commissioned by a conservative website, The Washington Free Beacon. But after Mr. Trump won the Republican nomination in the summer of 2016, the Democratic National Committee and lawyers working for Democratic nominee Hillary Clinton took over the funding - which was when the firm hired Mr. Steele.
Partisan tensions over releasing Mr. Simpson's interview transcript have been intense. Mr. Grassley has pushed to expand the panel's Russia probe into suspicions that the unverified dossier was used to justify the launch of the FBI's Trump-Russia investigation - in addition to helping secure a warrant to monitor former Trump adviser Carter Page.
Last week, Mr. Grassley and Judiciary Committee member Sen. Lindsey Graham announced they were issuing a criminal referral to the Justice Department, suggesting officials investigate Mr. Steele for possibly lying to the FBI. The move angered committee Democrats, who said they were not consulted ahead of the referral.
In a statement Tuesday after Ms. Feinstein released the transcript, Mr. Grassley called the move "totally confounding" and said it jeopardized "the panel's ability to secure candid voluntary testimony" from future witnesses.