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Hillary Clinton's presidential campaign and the Democratic National Committee helped pay for research that went into a salacious dossier on Donald Trump's alleged campaign ties to Russia, The Washington Post reported.
The payments continued until just days before the November 2016 election.
Even before that deal, Fusion GPS's research into Trump was funded by a rival in the Republican presidential primary election. That Republican's identity is not yet known.
Fusion GPS has asked that a federal judge to block a House Intelligence Committee request to gain access to the company's bank records, saying it would violate its constitutional rights.
The 35-page political research dossier, prepared by former MI6 officer Christopher Steele, has become central to congressional probes into Russian interference in the 2016 US elections.
Trump has branded the dossier -- which contains salacious, and partially unconfirmed, political, financial and sexual allegations -- as "fake news."
On Saturday, Trump tweeted that the Justice Department and FBI "should immediately release who paid for it".
Two days earlier, he mused publicly on Twitter that Democrats may have funded the research.
"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)?" the president wrote.