Washington(CNN) Special Counsel Robert Mueller's investigators met this past summer with the former British spy whose dossier on alleged Russian efforts to aid the Trump campaign spawned months of investigations that have hobbled the Trump administration, according to two people familiar with the matter.

Information from Christopher Steele, a former MI-6 officer, could help investigators determine whether contacts between people associated with the Trump campaign and suspected Russian operatives broke any laws.

CNN has learned that the FBI and the US intelligence community last year took the Steele dossier more seriously than the agencies have publicly acknowledged. James Clapper, then the director of national intelligence, said in a January 2017 statement that the intelligence community had "not made any judgment that the information in this document is reliable."

The intelligence agencies, particularly the CIA, and the FBI took Steele's research seriously enough that they kept it out of a publicly-released January report on Russian meddling in the election in order to not divulge which parts of the dossier they had corroborated and how.

This contrasts with attempts by President Donald Trump and some lawmakers to discredit Steele and the memos he produced.

Ever since the dossier came to light in January, Trump and his allies have repeatedly insisted that it is a complete work of fiction. He told The New York Times this summer that the dossier "was totally made-up stuff." In a series of tweets earlier this year, Trump said the memos were written by a "failed spy" who had relied on "totally made-up facts by sleazebag political operatives."

While the most salacious allegations in the dossier haven't been verified, its broad assertion that Russia waged a campaign to interfere in the election is now accepted as fact by the US intelligence community. CNN also reported earlier this year that US investigators have corroborated some aspects of the dossier, specifically that some of the communications among foreign nationals mentioned in the memos did actually take place.

At a news conference this week, Sen. Richard Burr, chairman of the Senate intelligence committee, expressed frustration that Steele has rebuffed efforts to meet with his committee.

"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?" the North Carolina Republican said. "We're investigating a very expansive Russian network of interference in US 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."

A representative for Steele did not respond to a request for comment. The special counsel's office declined to comment.

The dossier is a collection of memos that were initially intended as political opposition research, sources tell CNN. Steele was hired in the summer of 2016 by a Washington firm that was already collecting opposition research about Trump. The project was initially funded during the GOP primaries by anti-Trump Republicans, but Democrats started picking up the tab once Trump became the presumptive nominee in the spring.

In the weeks before the US intelligence community published a January report detailing Russian meddling efforts in the 2016 election, top officials at the FBI, CIA and the Office of the Director of National Intelligence discussed including parts of the Steele dossier in the official intelligence document, sources tell CNN.

The debate came in part because the FBI was concerned about being alone in shouldering the responsibility of briefing the incoming President about the allegations. FBI officials hopes that including the dossier allegations in the intelligence report would show the entire intelligence community speaking in one voice.

Then-FBI Director James Comey expressed concerns to his counterparts that if the FBI alone presented the dossier allegations, then the President-elect would view the information as an attempt by the FBI to hold leverage over him.

But the intelligence community had bigger concerns, sources tell CNN. The classified version of the report would be disseminated beyond then-President Barack Obama and the President-elect to other officials including members of Congress. And if that report included the dossier allegations, the intelligence community would have to say which parts it had corroborated and how. That would compromise sources and methods, including information shared by foreign intelligence services, intelligence officials believed.

In the end, the decision was made that the FBI and Comey personally would brief the incoming President on the allegations. That briefing occurred January 6 in a one-on-one conversation following a broader intelligence briefing on Russian meddling provided to then-President-elect Trump and his key staff.

Trump later told The New York Times in July that he took Comey's briefing on the dossier to be an attempt to hold it as leverage over the new President.

"In my opinion, he shared it so that I would think he had it out there," Trump said.

Exactly what Comey feared had come to pass.