Fusion GPS chief Glenn Simpson told a Senate panel previous year that a controversial dossier his firm produced alleging Donald Trump had suspicious connections with Russians was substantiated and that the document's author briefed the Federal Bureau of Investigation on his findings, according to a newly released transcript.
Fusion initially was hired through a conservative website, the Washington Free Beacon, during the Republican primaries to dig up dirt on Trump.
Grassley's office slammed Feinstein for releasing the testimony, saying it came as the panel was still trying to secure testimony from other witnesses, including Trump's son-in-law, Jared Kushner.
Mr. Simpson has also spoken with investigators from the House and Senate intelligence committees, which have their own investigations into Russia's election interference.
The dossier is a compilation of memos written by Steele during the 2016 campaign that contained allegations of connections between Trump and Russian Federation, including that Trump had been compromised by the Kremlin.
But in the late stages of the 2016 presidential campaign, Steele broke off contact with the FBI, Simpson said, because he did not believe it was taking him seriously enough.
I'm Proud We Published the Trump-Russia Dossier
"My understanding was that they believed Chris at this point - that they believed Chris might be credible because they had other intelligence that indicated the same thing and one of those pieces of intelligence was a human source from inside the Trump organization", Simpson said.
Mr Trump has repeatedly denied colluding with Russian Federation during the campaign, and called the entire investigation a "witch hunt".
Fusion GPS founder Glenn Simpson had asked that a transcript of his 10 hours of testimony before Senate Judiciary Committee staff be made public, Feinstein said. "Feinstein for her courage", the research firm said in a statement.
Feinstein's decision came after a tense back-and-forth between Republicans and Democrats over what to do with the transcript of Simpson's interview, which he provided on August 22.
Feinstein said she released the transcript to set the record straight.
Christopher Steele told the political research firm that hired him, Fusion GPS, that what he uncovered from Russian sources was serious enough to bring to the attention of US law enforcement authorities, according to a transcript released on Tuesday. Republican Sen. John Cornyn of Texas told reporters that the release was a "good idea" and that transparency is important as they work to understand the impact of the dossier. That decision marks the most serious break yet in the cooperative relationship she has had with the Republican chairman of the committee, Sen. Chuck Grassley, issued a criminal referral last week for Steele, alleging the committee had reason to believe that the former spy had lied to authorities about his conversations with the press regarding the dossier.