An attorney for the co-founder of opposition research firm Fusion GPS revealed during a closed-door interview this summer with congressional investigators that “somebody's already been killed” as a result of the publication of the anti-Trump dossier.

The statement was contained in a 312-page transcript of Fusion GPS co-founder Glenn Simpson's August interview with committee investigators, released unilaterally Tuesday by Senate Judiciary Committee Ranking Member Dianne Feinstein, D-Calif.

The release itself provoked controversy, with an aide to Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, calling it “confounding” for Feinstein to drop the transcript in the “middle of an ongoing investigation.”

Among the many revelations in the document, though, is the claim from Simpson's attorney that someone died because the dossier -- which was commissioned by Fusion GPS -- was publicly released.

“He wants to be very careful to protect his sources,” attorney Josh Levy said during the Aug. 22 Senate Judiciary Committee interview of his client. “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.”

Glenn Simpson's attorney said someone has been killed over the Trump dossier's release.

Levy didn't elaborate on who was killed. The website BuzzFeed first published the dossier online last January, airing its unverified allegations about President Trump's connections with Russia.

The dossier was written by former British MI6 agent Christopher Steele. Fusion GPS, which hired Steele, got funding from the Clinton campaign and Democratic National Committee. The findings eventually made their way to the FBI -- and the bureau's dealings with Steele are a key focus for congressional investigators.

During his interview, Simpson described his conversations with Steele about their decision to turn the dossier details over to the bureau.

“He thought from his perspective there was an issue -- a security issue about whether a presidential candidate was being blackmailed,” Simpson said.

Simpson recalled telling Steele he had to think about whether they should contact the FBI, saying he wasn’t sure who to give it to.

Sen. Dianne Feinstein stunned Sen. Chuck Grassley by releasing the transcript from Glenn Simpson's interview. (AP)

“In any event, he said, ‘Don't worry about that, I know the perfect person, I have a contact there, they'll listen to me, they know who I am, I'll take care of it,’” Simpson recalled.

Steele also told Simpson the FBI had “an internal Trump campaign source,” he said.

“They believed Chris's information 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 of the FBI.

Simpson, a journalist before forming Fusion GPS, also admitted in the interview being opposed during the election to Trump becoming president.

“I think it's safe to say that, you know, at some point probably early in 2016 I had reached a conclusion about Donald Trump as a businessman and his character and I was opposed to Donald Trump,” Simpson said.

Trump has long derided the dossier as inaccurate, and several GOP-led committees are investigating whether it formed the basis for the FBI's initial investigation into Russian election interference.

Feinstein said in a statement she released the transcript to combat misinformation about the interview.

“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,” Feinstein said in a statement. “The only way to set the record straight is to make the transcript public.”

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Taylor Foy, a spokesman for Grassley, said Feinstein’s release “undermines the integrity of the committee’s oversight work and jeopardizes its ability to secure candid voluntary testimony relating to the independent recollections of future witnesses.”

“It’s totally confounding that Senator Feinstein would unilaterally release a transcript of a witness interview in the middle of an ongoing investigation – a witness that Feinstein herself subpoenaed last year for lack of cooperation,” Foy said.

Fox News’ Jake Gibson, Cody Derespina and Adam Shaw and The Associated Press contributed to this report.