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A law firm representing the Hillary Clinton campaign and the Democratic National Committee helped fund opposition research that eventually became a controversial dossier on then-candidate Donald Trump, a source confirmed to NBC News Tuesday.

The Washington Post, which first reported the funding of the research earlier Tuesday, said it shows the Clinton campaign and the DNC helped pay for what would become the dossier that contained unverified and salacious allegations about Trump.

"To aid in its representation of the Clinton presidential campaign and the Democratic National Committee, [the law firm] Perkins Coie retained [intelligence firm] Fusion GPS, entering into an engagement for research services that began in April 2016 and concluded before the election in early November," a source familiar with the situation told NBC News.

Perkins Coie represented the Clinton campaign and the DNC at the time.

Trump has strongly denied the allegations in the dossier, which alleged that the Trump campaign had colluded with Russia. It also contained other salacious allegations, which Trump has vigorously denied, and some of which have been debunked.

White House Press Secretary Sarah Huckabee Sanders Tuesday night tweeted from her official account: "The real Russia scandal? Clinton campaign paid for the fake Russia dossier, then lied about it & covered it up."

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The Washington Post reported that before Perkins Coie retained Fusion GPS on behalf of the Clinton campaign and the DNC, Fusion's research was funded by an unknown Republican client during the GOP primary.

A spokesperson for the DNC said in a statement that the organization's new chairman and leadership was not involved in the Fusion GPS matter.

"[Chairman] Tom Perez and the new leadership of the DNC were not involved in any decision-making regarding Fusion GPS, nor were they aware that Perkins Coie was working with the organization," DNC Communications Director Xochitl Hinojosa said in the statement. "But let's be clear, there is a serious federal investigation into the Trump campaigns ties to Russia, and the American public deserves to know what happened."

Before Tuesday's report, sources familiar with the situation had told NBC News that while the research had been initially funded by an unknown Republican organization, an unknown Democratic group later took over the funding.

Christopher Steele, the former MI6 agent who set-up compiled a controversial dossier on Donald Trump, seen on March 7, 2017, in London. Victoria Jones / PA Wire/PA Images

The paper, citing sources familiar with the matter, reported that Fusion GPS hired former British intelligence officer Christopher Steele and turned Steele's documents over to lawyer Marc. E. Elias, who is with Perkins Coie, and that it was unclear how much of that was shared with either the Clinton campaign or the DNC.

Former Clinton campaign press secretary Brian Fallon said Tuesday on Twitter: "I regret I didnt know about Christopher Steele's hiring pre-election. If I had, I would have volunteered to go to Europe and try to help him."

The Washington Post reported that the Clinton campaign and the DNC shared the cost. A Clinton campaign spokesperson had no immediate comment.

According to the Post, a source "close to the matter" said the Clinton campaign and the DNC were told of Fusion GPS’s role by the law firm.

Elias and Perkins Coie did not return requests for comment from NBC News.

The dossier was published by Buzzfeed in January. NBC News reported earlier this month that members of special counsel Robert Mueller's team, which is investigating any connections between the Trump campaign and Russia, have traveled to interview Steele.

Related: Who is Glenn Simpson, Man Entangled in Two Russia Scandals?

Trump has denounced the document and has denied that any collusion took place.

House Intelligence Committee chair Rep. Devin Nunes, R-California, issued subpoenas for Fusion GPS earlier this month, people familiar with the matter told NBC News.

Steele's sources provided the bulk of the dossier, but Fusion co-founder Glenn Simpson and his partners were involved in the investigation, people familiar with the matter have told NBC News.