Hillary Clinton's campaign and the Democratic National Committee helped bankroll research that led to the infamous dossier on Donald Trump.
Clinton's campaign lawyer Marc Elias hired research firm Fusion GPS back in April 2016 to look into allegations of Trump's ties to Russia, according to the Washington Post.
Fusion GPS, the Washington-based research firm, then hired former British intelligence officer Christopher Steele to dig up the unconfirmed dirt on Trump.
Clinton's lawyer and his law firm Perkins Coie continued to fund the research until October 2016 - just days before the presidential election.
The research was previously funded by an unknown anti-Trump Republican donor during the primary, but Clinton's campaign then paid for it to be finished.
Sources would not confirm how much was paid to Fusion GPS, but said the campaign and DNC shared the cost.
Steele's findings and research were then submitted to Elias via Fusion GPS, the Post reports. It is not clear what information, or how much of it, was provided to Clinton's campaign. It also isn't clear which people within Clinton's campaign and the DNC knew of Steel and Fusion GPS.
The research resulted in the now infamous dossier that came up with the discredited claims about Trump. The dossier has become a focus of congressional probes into Russian interference in last year's election.
Trump tweeted on Saturday that the FBI and Justice Department should 'immediately release who paid for it' after Fusion GPS co-founder Peter Fritsch and partner Thomas Catan last week invoked the US Constitution's Fifth Amendment, which protects Americans against self-incrimination.
The two men had been subpoenaed to appear in a closed-door session before the House Intelligence Committee but refused to speak at every turn.
Committee chair Rep. Devin Nunes has been trying to determine who paid Fusion GPS for the opposition research that formed the basis of the dossier.
"Officials behind the now discredited 'Dossier' plead the Fifth. Justice Department and/or FBI should immediately release who paid for it," Trump tweeted.
Trump had also speculated on Twitter last week that the dossier may have been funded by Democratic officials.
"Workers of firm involved with the discredited and Fake Dossier take the 5th. Who paid for it, Russia, the FBI or the Dems (or all)?" Trump said last Thursday.
Fusion GPS, Elias and spokesmen for Clinton's campaign and the DNC would not comment.
Prior to the Post report, Elias had earlier "vigorously" denied being involved in the Trump dossier, according to New York Times reporter Ken Vogel.
"When I tried to report this story. Clinton campaign lawyer (Marc Elias) pushed back vigorously, saying 'You (or your sources) are wrong'," Vogel tweeted shortly after the Post report surfaced.
The dossier contends that the Russian government amassed compromising information about Trump but had also been engaged in a years-long effort to support and assist him.
Compiled by the British spy, Christopher Steele, the dossier circulated among Washington journalists last year until Buzzfeed first published it in January.
Trump branded the entire document "fake news".
Among its claims are that Russian officials have videos of the president cavorting with prostitutes, which were filmed during Trump's 2013 visit to a luxury Moscow hotel for the Miss Universe contest
It also contains a highly unusual report, also never substantiated, that the call girls performed a fetish act on a hotel bed.
THE TRUMP-RUSSIA DOSSIER TIMELINE:
The dossier is first dated June 20 and had contained several unverifiable periodic reports made over the summer, according to Mother Jones. It was sent in dated sections from a former Western intelligence officer to the FBI and alleged Russia had enough to blackmail Trump.
It alleged that Trump had been cultivated by Russian officials "for at least five years". It also claimed that the Kremlin had compromising material related to "sexually perverted acts" Trump performed at a Moscow Ritz Carlton where former President Barack Obama once stayed.
Dossier also alleged that Trump's inner circle was accepting a regular flow of intelligence from the Kremlin on Hillary Clinton.
July 27: Trump asks Russian hackers to find Clinton's 30,000 emails during a press conference.
July 31: Kremlin weighing to release more information about Clinton.
Late July: The FBI opens its investigation into Russia's interference in the election, and the Trump campaign's possible role in it.
August 27: Then-US Sen. Harry Reid sent a letter to then-FBI Director James Comey and called for a full investigation and public disclosure. He wrote: "The evidence of a direct connection between the Russian government and Donald Trump's presidential campaign continues to mount and has led Michael Morrell, the former Acting Central Intelligence Director, to call Trump an 'unwitting agent' of Russia and the Kremlin."
September 23: US intelligence officials began investigating links between Trump adviser Carter Page and the Russian government, Yahoo News reported. Page had extensive business links in Russia and is a former Merrill Lynch investment banker in Moscow.
October 7: The Obama administration publicly accuses Russia of "directing the recent compromises of emails from US persons and institutions, including from US political organisations' to affect the US election.
October 30: Reid sent Comey another letter demanding that Trump's possible ties to Russia be fully investigated and he cited the existence of "explosive information" that the FBI has in its possession.
November 3, 2016: Russian oligarch Dmitry Rybolovlev flies into Charlotte, North Carolina on a private plane. Trump's plane lands on the tarmac not long after and parks next to Rybolovlev, whose plane stays in Charlotte for 22 hours afterward. Trump rallies in nearby city Concord.
November 8: Trump wins the election to become the 45th president of the United States.
November 10: President Barack Obama warns Trump during a meeting at the White House that national security adviser Michael Flynn, a former US Army lieutenant general and Defense Intelligence Agency chief, is a problem.
November 18: During a security meeting in Halifax, Nova Scotia, Sen. John McCain hears about the documents and dispatched a former US official to meet the source of the documents and gather more information.
December 9: McCain meets Comey and gives the FBI director the documents, The Guardian reported.
December 13: This is the last date of the memos from the dossier written by the British source.
December 29: The Obama administration issues new sanctions on Russia in retaliation for Russia's hacking of the Democratic National Committee in the summer and other efforts to interfere with the US election.
January 10: Obama and Trump were both given a two-page summary of the dossier, CNN reported. BuzzFeed News then reported on the dossier and published it in full about how it allege Trump's deep ties with Russia.
January 19: The New York Times reported that "intercepted communications" between Trump associates and Russians are being investigated as part of the FBI's inquiry into Russia's election meddling.
January 27: Trump's lawyer, Michael Cohen, holds a meeting with Russian-American businessman Felix Sater and Ukrainian lawmaker Andrii Artemenko to discuss a backchannel "peace plan" for Russia and Ukraine.
February 13: Flynn resigns as national security adviser after reports emerge that he misled Vice President Mike Pence.
March 2: Attorney General Jeff Sessions recuses himself from the investigation into whether the Trump campaign communicated with Russia.
March 4: Without presenting evidence, Trump tweets that Obama had Trump Tower's "wires tapped" during the presidential campaign.
March 15: Rep. Devin Nunes, the chairman of the House Intelligence Committee, announced that the committee had not found any evidence to support Trump's wiretapping claim.
March 20: Comey said he has "no evidence" to support Trump's wiretapping claim. He confirmed that an investigation into Russia's election-related meddling includes an examination of contacts between Trump associates and Russia during the campaign.