The list of witnesses to have been interviewed so far is long, and includes Donald Trump Jr and Jared Kushner, as well as lesser figures such as former adviser Carter Page; Glenn Simpson, the co-founder of Fusion GPS, which commissioned the Steele dossier; and Ben Rhodes, the former Obama adviser.
Glenn Simpson, the founder of Fusion GPS, was questioned in August 2017 by Judiciary Committee investigators about the dossier commissioned by his firm, which contained salacious and unverified material on President Donald Trump's purported ties to Russian Federation.
In the interview, Simpson was asked if he had taken steps "assess the credibility" of the sources for Steele's dossier, but he declined to discuss "sourcing information". "One of those pieces of intelligence was a human source from inside the Trump organization".
Fusion initially was hired through a conservative website, the Washington Free Beacon, during the Republican primaries to dig up dirt on Trump.
This private interrogation of the head of the Fusion GPS research firm was released in a transcript Tuesday, dumped onto the internet by a high-ranking Democrat who expressed frustration over how that material is being treated by her partisan adversaries.
Steele's dossier uses information gleaned from close associates of Sechin's.
Mr Steele was said to have passed on his findings to both U.S. and United Kingdom intelligence in the run-up to the 2016 presidential election due to concerns about national security for both countries.
There are other variations on this theme, but central to all of them was the idea that without Fusion there would be nothing, and that we would know this if only we could know what Simpson said to the Senate investigators. "Like if you're a lawyer and, you know, you find out about a crime, in a lot of countries you must report that". Simpson said his spy-for-hire anxious that the president-to-be risked being blackmailed by familiar Russian espionage tactics.
Several people familiar with the probe said Simpson's comments refer to a report from an Australian official who contacted US officials in late July with concerns about a conversation months earlier in London with Trump campaign adviser George Papadopoulos.
After the article was published, Simpson said, Steele severed his relationship with the Federal Bureau of Investigation.
"We would like to get Jared Kushner before, and you can understand why he might raise a question on whether he should come up here and answer questions for us if it's going to be released without anybody's knowledge", Grassley said, referring to Trump's son-in-law.
"The dossier is, and continues to be, the subject of active investigations by Congress and intelligence agencies".
Steele's dossier was published in full by Buzzfeed in January 2017 after being offered to numerous news outlets.
He said he found it weird that Trump kept claiming he didn't know Sater well.
"It's a false allegation", he said. The White House has said it believes Mr Mueller is indeed reaching the end of his work and Mr Trump is said to have told friends he expects to be "cleared in writing". The memos are field reports of real interviews that Chris's network conducted and there's nothing phony about it.
"It was someone like us who made a decision to pick up the phone and report something", Simpson said of the source.
At the time RPC, the United Kingdom law firm representing Mr Steele, declined to comment. But he testified that the two cases were entirely separate, and did not influence one another.
But what about Trump officials meeting with the Russians during the campaign?