Glenn Simpson, a founder of the research firm Fusion GPS, spoke to investigators with the Senate Judiciary Committee for 10 hours in August.
Fusion GPS founder Glenn Simpson, who previously worked as a reporter at the Wall Street Journal, had asked that a transcript of his 10 hours of testimony in August before Judiciary Committee staff be made public, Feinstein said.
Simpson of Fusion GPS, took place last summer and was expected to shed light on the origins of the firm's work, its concerns about the Trump campaign's activities, and what the F.B.I. may have done with the information.
"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", Simpson said. Simpson has denied that it did and, according to the transcript, told investigators that the Federal Bureau of Investigation told Steele that the government also had intelligence from "an internal Trump campaign source".
"The only way to set the record straight is to make the transcript public", she said in a statement. The firm commissioned the dossier, which was initially paid for by a conservative website and then later by Democrats, including Hillary Clinton's presidential campaign.
Mr. Simpson has also spoken with investigators from the House and Senate intelligence committees, which have their own investigations into Russia's election interference.
Feinstein said Democrats supported releasing the transcript, CNBC reports.
"Her action undermines the integrity of the committee's oversight work and jeopardizes its ability to secure candid voluntary testimony", said Grassley spokesman Taylor Foy.
It wasn't clear from the transcript, because of the differing ways Simpson described the source, whether the person was part of Trump's business or his campaign.
"I understand Chris severed his relationship with the FBI out of concern that he didn't know what was happening inside the FBI and there was a concern that the FBI was being manipulated for political ends by the Trump people", Simpson told the Senate Judiciary Committee. However, a person close to the investigation said Fusion GPS has long anxious that Steele's sources could be in danger, given a handful of killings that took place in the months after the dossier's existence became known.
Democratic Senator Dianne Feinstein released a transcript of the interview on Tuesday, after repeated disagreements with Republicans on the committee.
Democrats on the committee said they were not consulted ahead of the Republicans' referral to the Department of Justice. Democrats say such attacks are an effort to discredit the ongoing probe by special counsel Robert Mueller into whether any Trump associates coordinated with Russian agents to interfere in the presidential election.