That would be the dubious dossier put together by a former British intelligence agent, Christopher Steele during the primary season.
The dossier was originally commissioned by Republican challengers (I’ve heard either Jeb Bush or John McCain, but both deny it), then picked up by Democrats, once Trump was named the GOP nominee.
The dossier became public knowledge when Buzzfeed News decided to publish some of the more salacious portions without substantiation.
While most details remain unsubstantiated, there was enough there to raise the interest of those who have seen Trump’s expressed affection for Russia, and Russian President Vladimir Putin as troublesome.
Add that to a documented Russian influence campaign during the 2016 election – with a noted preference for Trump over Clinton – then you’ve got enough kindling to ignite an investigation.
The Senate Intelligence Committee has been vying to talk with Steele, with no luck, but it seems Robert Mueller’s team got him as early as this past summer, and they’ve been examining that dossier.
Although not publicly stated, the FBI and the U.S. intelligence community in 2016 regarded the dossier very seriously. Various intelligence agencies, especially the CIA and the FBI, regarded the dossier so seriously, they prevented it from being included in a publicly released report in January that discussed Russia’s interference in the 2016 election to prevent revealing which parts of the dossier they had verified.
However, Trump has greatly downplayed the dossier and said it “was totally made-up stuff” to the New York Times.
Some aspects of the dossier — like communications between foreign nationals noted in the dossier — have been confirmed by officials; however, the majority of the scandalous allegations included in the document have not been verified.
Senator Richard Burr (R-N.C.) has expressed the frustration of the Senate Intelligence Committee over what they see as an inability to move forward, without the benefit of talking to Mr. Steele about his work.
“The committee cannot really decide the credibility of the dossier without understanding things like, who paid for it? Who are your sources and sub-sources?” Burr said. “We’re investigating a very expansive Russian network of interference in U.S. elections. And though we have been incredibly enlightened at our ability to rebuild backwards, the Steele dossier, up to a certain date, getting past that point has been somewhat impossible.”
How important the dossier is to the committee or Mueller’s team may depend on how each are approaching the investigation.
We know the Mueller team are more likely focused on a criminal investigation, with reports that they’re looking into money laundering and issues along that vein.
At the very least, some of the creepier details of the dossier could be ruled out and put to rest.