The information was revealed in testimony released Tuesday.

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A transcript of Senate testimony by Fusion GPS founder Glenn Simpson says the FBI had a source inside then-presidential candidate Donald Trump's election campaign.

Simpson testified that the author of the Trump dossier, Christopher Steele, was concerned about whether Trump was being blackmailed.

The transcript reads, "He honed in on this issue of blackmail as being a significant national security issue."

Republicans have been trying to undermine the credibility of the document, saying Hillary Clinton's campaign partially funded the document to discredit Trump.

But according to Simpson's testimony, Steele thought the FBI would deem his claims credible because its person inside the campaign would corroborate them.