The co-founders of Fusion GPS downplayed former White House national security aide Fiona Hill's impeachment testimony after she called the anti-Trump dossier they commissioned a "rabbit hole."

Hill, a Russia expert who served on President Trump's National Security Council, testified that the controversial dossier “very likely” contained Russian disinformation, and its author, British ex-spy Christopher Steele “could have been played” by the Russians.

Glenn Simpson and Peter Fritsch, former Wall Street Journal reporters who co-founded Fusion GPS, were pressed to respond Sunday during an interview on NBC News's Meet the Press. Host Chuck Todd began by presenting video of Hill's recent public testimony and calling her an "expert."

Simpson rejected Todd's premise, asserting that Hill "is not of the same exact area of expertise" as Steele, a former head of MI6's Russia desk who now runs a private intelligence firm in London.

"She is not a disinformation specialist, and so she is a Russia specialist in general, she is entitled to her opinion. I know she knows Chris and has worked with him for a long time. So, I am not sure that that is very well understood in that one remark," he added.

Fritsch said he was "puzzled" by Hill's "rabbit hole" comment.

"I don't really know what a rabbit hole means in that context," he said. "I will say, though, that you can't actually catch rabbits without going down some rabbit holes. So you need to actually follow leads where they take you. And we have full confidence in Chris's ability to do exactly that."

Steele's dossier contained claims that Russia possessed damaging information about Trump, including a so-called pee tape, that could be used as blackmail. BuzzFeed published the dossier in January 2017 while noting it could not verify or disprove the claims.

The FBI relied on the dossier to obtain warrants to wiretap onetime Trump campaign adviser Carter Page, who was suspected to be a Russian asset but was never charged. The Foreign Intelligence Surveillance Act application and three renewals targeting Page are now the subject of a forthcoming report from the Justice Department watchdog, due out on Dec. 9.

Former FBI Director James Comey defended the bureau's use of the dossier, despite it being unverified, arguing that its findings were “consistent with our other intelligence." Although former special counsel Robert Mueller undercut two of the biggest claims in Steele's dossier, his report corroborated its central tenet: That Russia sought to interfere in the 2016 election to undermine Hillary Clinton and boost Trump.

Simpson and Fritsch have a book on Steele's dossier coming out on Tuesday, titled Crime in Progress: Inside the Steele Dossier and the Fusion GPS Investigation of Donald Trump. Simpson acknowledged that the book touches on some lingering questions about Steele, despite their confidence in his work.

"There are certain things we wonder about," he said, referring to the allegations in Steele's dossier.

Republicans claim the dossier is evidence that Democrats, not Trump, conspired with Russia during the 2016 campaign. "The Democrats colluded with Russian sources to develop the Steele dossier," House Intelligence Committee ranking member Devin Nunes said during a hearing with Mueller in July.

But Simpson argued the "Democratic conspiracy" is baseless, saying, "It doesn't make logical sense for it to be a Democratic conspiracy if half, if the first half of the investigation was paid for by Republicans."

The conservative Washington Free Beacon bankrolled Fusion GPS to investigate Trump and other Republican candidates during the 2016 GOP primaries but dropped the project after Trump locked up the nomination. But further funding for their research came from Clinton's campaign and the Democratic National Committee, through the Perkins Coie law firm, and Steele was hired to compile the dossier.

Fusion GPS continues to investigate into Russian election interference, but Simpson rejected Todd's suggestion that they are "actively" investigating Trump.

Simpson asserted that the investigation is more broadly "investigating Russia's efforts to affect and disrupt western democracies" and acknowledged they are receiving funding from the Democracy Integrity Project, a group backed by liberal billionaire George Soros that was founded in 2017 by a former staffer to Democratic California Sen. Dianne Feinstein.