The head of the research firm behind a dossier of allegations against then-presidential candidate Donald Trump told congressional investigators that someone inside Trump’s network had also provided the FBI with information during the 2016 campaign, according to a newly released transcript, a claim quickly disputed by people close to the investigation into Russian interference in the election.

Glenn Simpson, a founder of the research firm Fusion GPS, spoke to investigators with the Senate Judiciary Committee for 10 hours in August. As the partisan fight over Russian interference in the 2016 election has intensified, Simpson has urged that his testimony be released, and a copy of the transcript was made public Tuesday.

It was released by the ranking Democrat on the Senate Judiciary Committee, Sen. Dianne Feinstein of California. That decision marks the most serious break yet in the cooperative relationship she has had with the Republican chairman of the committee, Sen. Charles E. Grassley, of Iowa.

A spokesman for Grassley called Feinstein’s move “totally confounding” and done without consultation. “Her action undermines the integrity of the committee’s oversight work and jeopardizes its ability to secure candid voluntary testimony relating to the independent recollection of future witnesses,” said the spokesman, Taylor Foy.

Feinstein said she released the transcript to set the record straight. “The innuendo and misinformation circulating about the transcript are part of a deeply troubling effort to undermine the investigation into potential collusion and obstruction of justice,” she said.

A representative for Fusion GPS did not immediately offer a comment.

Fusion GPS was hired in mid-2016 by a lawyer for Hillary Clinton’s presidential campaign and the Democratic National Committee to dig into Trump’s background. Earlier that year, the firm had been probing Trump for a conservative website funded by a Republican donor, but that client stopped paying for the work after it became clear Trump would win the Republican nomination, according to people familiar with the matter.

After Democrats began paying for the research, Fusion GPS hired Christopher Steele, a former senior officer with Britain’s intelligence service, MI6, to gather intelligence about any ties between the Kremlin and Trump and his associates. Steele’s reports were eventually compiled into a dossier alleging the Trump campaign coordinated with the Kremlin – a claim the president has repeatedly denied.

Steele first reached out to the FBI with his concerns in early July 2016, according to people familiar with the matter. When they re-interviewed him in early October, agents made it clear, according to Simpson’s testimony released Tuesday, that they believed some of what Steele had told them.

“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.

Simpson said he didn’t know whether the person was connected to the Trump campaign or a Trump company, adding that his understanding was the source was someone who had volunteered information to the FBI or, in his words, “someone like us who decided to pick up the phone and report something.”

One person familiar with the probe said Simpson’s comments misrepresent what had actually happened – that it was an Australian official who reached out to the United States in late July with concerns about a conversation months earlier in London with Trump campaign adviser George Papadopoulos. Papadopoulos has since pleaded guilty to lying to the FBI and is cooperating with investigators.

During Simpson’s interview in August, a Republican staffer pressed him further on this claim, and Simpson’s answers were vague. Steele “would say very generic things like I saw (the FBI), they asked me a lot of questions, sounds like they have another source or they have another source. He wouldn’t put words in their mouth,” the transcript says.

At another point in the interview, a lawyer for Fusion GPS, Joshua Levy, makes a jarring assertion: that the dossier’s publication had led to someone’s murder.

“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,” Levy said late in the interview, according to the transcript.

Levy did not expand on that claim in the interview, nor is there any public information that would tie a specific killing to information in the dossier. However, a person close to the investigation said Fusion GPS has long worried 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.

In recent weeks, as the political fights about the Russia investigation and the dossier have intensified, Simpson has urged the committee to release the full transcript of his interview, arguing that Republicans are trying to obscure, rather than reveal, what happened in 2016.

Through much of 2017, Feinstein and Grassley made joint requests for information about Russia and the FBI’s investigation of election interference. In the fall, however, tensions between Grassley and Feinstein spilled out into the open as Grassley requested information from the FBI and other sources without Feinstein’s support.

Increasingly, the Democrats and Republicans on the committee are going in different directions – with Grassley moving to investigate matters involving Clinton when she was secretary of state and Feinstein concentrating on Russian interference in the 2016 election.

While Simpson has accused conservative lawmakers of acting in bad faith, Republicans have accused Steele, while working for Fusion GPS, of misleading the FBI. Last week, Grassley made a criminal referral to the Justice Department, suggesting Steele may have lied to the FBI. While details of the referral are classified, it appears to be related to Mr. Steele’s contacts with reporters during the election campaign.

Republicans have attacked the credibility of Steele’s dossier. Democrats say such attacks are an effort to discredit the ongoing probe by special counsel Robert S. Mueller III into whether any Trump associates coordinated with Russian agents to interfere in the presidential election.

Send questions/comments to the editors.