FBI Director James Comey considered an anti-Trump dossier compiled by a former British intelligence officer so important that he insisted the document be included in January's final intelligence community report on Russian meddling in the U.S. election, Fox News was told.

Sources would not speak on the record, citing the sensitivity of the matter and its current relevance to upcoming testimony on the unmasking of American citizens as part of the FBI probe into alleged contacts between Trump campaign advisers and Moscow.

Asked for a response to the claims, the FBI and Office of the Director of National Intelligence said they could not comment on a classified document.

It was reported last month that the unverified dossier was part of the evidence the FBI used to obtain a FISA warrant for Carter Page, a peripheral figure in the Trump campaign. In an interview with Fox News, Page denied the dossier’s central allegations that he was the Trump campaign’s point person for Moscow.

The dossier also contained salacious allegations about then-candidate Donald Trump. The classified version of the intelligence report issued at the end of the Obama administration included a summary of the document, as an attachment. Both then-President Barack Obama and President-elect Trump were presented with the findings.

In a remarkable exchange Wednesday between Sen. Lindsey Graham, R-S.C., and Comey during a Senate hearing, more questions were raised about the bureau’s relationship with the former British officer who wrote the dossier, Christopher Steele, and his British company Orbis -- and its relationship to Washington, D.C.-based Fusion GPS.

Comey said that “I know the name” and “I can’t say” when Graham pressed if Fusion GPS is part of the Russian intelligence apparatus.

And when Graham asked if he agreed it “would be interfering in our election by the Russians” if Fusion were involved in preparing the anti-Trump dossier, Comey replied, “I don’t want to say.”

Inside the packed hearing room, the committee’s chairman Sen. Charles Grassley, R-Iowa, was visibly irritated at the lack of response by the director of the FBI. For months, Grassley has highlighted the bureau’s failure to meet deadlines and respond to his questions, calling it in a recent letter a “pattern of obstruction” on matters related to Steele.

Sen. Dianne Feinstein, D-Calif., the top Democrat on the Senate Judiciary Committee who also sits on the intelligence committee, told CNN on Wednesday that she does not “at this time” have evidence of collusion between Trump associates and Russia during the campaign.

A letter of complaint against Fusion GPS was filed with the Justice Department, alleging the company -- and founder former Wall Street Journal reporter Glenn Simpson -- violated the Foreign Agents Registration Act.

Asked for comment on the complaint’s allegation that Fusion GPS acted as an unregistered agent for Russian interests, attorney Joshua A. Levy told Fox News’ senior executive producer Pamela Browne via email that, “Fusion GPS has complied with the law and was not required to register under FARA. Nor has Fusion GPS ever been a Russian agent. Allegations to the contrary are not true.”

Levy went on to deny Fusion GPS lobbied against the Magnitsky Act. “Not true,” Levy stated.

He said: “Fusion GPS’ only factual connection to anything in the complaint is the litigation support Fusion GPS provided to Baker Hostetler, a law firm mentioned in the complaint. Fusion GPS did no lobbying work for that law firm or its clients.”

The Magnitsky Act, a 2012 bipartisan bill passed by Congress and signed by then-President Barack Obama, was named after Russian lawyer-turned-whistleblower Sergei Magnitsky, who died in a Moscow prison in 2009. The law imposes visa bans and asset freezes on Russian officials linked to the 37-year-old lawyer’s death.

Grassley stressed his concerns that “the FBI has relied on the document to justify [Comey's] current investigation. There have been reports that the FBI agreed to pay the author of the dossier, who paid his sources, who also paid their sub sources. Where did the money come from and what motivated the people writing the checks?”

During the heated hearing, Grassley also chided the FBI for giving him “materially inconsistent” information and specifically referenced Steele, noting “The man who wrote the dossier admitted in court that it has unverified claims. Does that sound like a reliable basis for law enforcement or intelligence actions?”

Fox News also spoke to Steele’s solicitor Nicola Cain in London, who had no comment citing ongoing litigation.

A British court document, first reported by The Guardian and signed by Steele, offers a glimpse into his company’s work for Fusion GPS. The document describes "unsolicited intelligence" and "raw intelligence" that “needed to be anlaysed and further investigated/verified.”

Fox News has filed a formal request for copies of court documents with the Queen’s Bench Division of the High Court in London.