The investigation into Russian intelligence activities by the Senate Select Committee on Intelligence turned two years old, without fanfare, last month.

For almost as long, the inquiry, led by Republican Chairman Richard Burr of North Carolina and Vice Chairman Mark Warner, a Virginia Democrat, has been held up as the last bastion of bipartisanship in Washington.

After a parallel investigation divided the House Intelligence Committee last year, the Senate's probe has been under intense pressure to offer a single set of findings.

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Burr, who is known in Senate hallways for his preference to go sockless and the two-fingered hook that often bears his jacket, has spoken little about the probe he leads. But he thinks deeply about how its conclusions should be presented. And he acknowledges now that the investigation is broader, and perhaps more consequential, than it has long been thought to be.

Committee Chairman Sen. Richard Burr arrives for a meeting of the Senate Select Committee on Intelligence on Aug. 16, 2018, in Washington, D.C. Win McNamee / Getty Images

"I'm not going to tell you that what we set out to do — which was to understand what happened in '16 — is what's extended the life of the investigation," Burr said in a rare interview with CBS News. "I think it's a better understanding of what happened and how coordinated and organized the effort was."

He spoke in his second-floor chambers in the Russell Senate Office Building, where multiple deer mounts circle the ceiling and a non-working fireplace is stacked with real wood. Burr, 63, had angled himself in a worn leather chair, legs extended, and tented his fingers around the edges of a coffee cup. He spoke quietly, with occasional long pauses as he considered his responses.

"We'll be judged at the end of this on the product that we produce," he said solemnly. "We'll also be judged on the process that we chose ... None of us ever anticipated that this would be two years."

For more than an hour, Burr detailed the committee's work and findings to date, explained why its investigation will stretch beyond its second year, and addressed the potential of a partisan breakdown at its conclusion. He described the committee's coordination with the investigation led by special counsel Robert Mueller, its plans for delivering a final report, and hinted at what kinds of questions it may, at least for now, have to leave unanswered.

He made clear that the investigation is not compiling the story of one pivotal election, but of something larger, more complicated and, from a counterintelligence perspective, more nefarious. The final report may be so highly classified, he said, that a meaningful portion may not be made public at all.

"Many of the connections that we've made are the direct result of intelligence products," he said. "I think it's safe to say we've interviewed people that I don't even know if the special counsel knows about them — but you've got to remember that we're on a totally separate path than what they are."

He acknowledged that a core part of committee's charge is to tell the country, in the greatest detail possible, what happened in 2016.

"The other piece of that," he said, "is probably work that the committee will do for the next decade. And it's work that has helped even our intelligence community's understanding of Russia's capabilities and intent behind this."

"This was not," he stressed, "'Let's go screw with the Americans in 2016.'"

Where it began

The committee's investigation was first announced in a joint statement from Burr and Warner on January 13, 2017 — one week before President Trump was inaugurated.

"The Committee will follow the intelligence wherever it leads," the joint announcement laying out its parameters read. "We will conduct this inquiry expeditiously, and we will get it right."

Burr, a Charlottesville native, had been named a national security adviser to the Trump campaign in October — as it happened, less than an hour before the "Access Hollywood" tape became public, and soon after the Obama administration released its first statement on Russia's election interference efforts.

Senate Intelligence Committee Chairman Richard Burr and ranking member Sen. Mark Warner hold a news conference about the committee's investigation into Russian interference into the 2016 presidential election at the Capitol on March 29, 2017, in Washington, D.C. Chip Somodevilla / Getty Images

His proximity to the Trump campaign and early statements suggesting it would be off-limits in the investigation initially alarmed some Democrats and prompted calls for an independent commission. Some observers urged the hiring of professional investigators.

After some behind-the-scenes wrangling, the political campaigns became part of the inquiry's scope, but Burr held firm that the probe would be driven by the committee's own staff. In a sign of Democrats' early unease, Warner said he would seek out other solutions if the committee demonstrated it could not "properly conduct" the investigation.

In subsequent months, Burr's defense of former FBI Director James Comey after his firing and his pushback against Mr. Trump's claims of having been "wiretapped" by the Obama administration helped grow perceptions that the committee's efforts would be bipartisan.

Burr said he felt vindicated by his decision to empower the committee's staff to run the investigation. He said their access to highly classified intelligence from the agencies the committee is equipped to oversee often allowed them to know in advance what they needed to elicit from a witness.

Outside investigators, Burr said, "would've never had access to some of the documents that we were able to access from the intelligence community."

In some cases, he said, it was "a precedent of information-sharing that had not ever existed in the history of the country."

"It also gave us tremendous insight to know when somebody was lying to us," he said, adding that the committee had "not been shy" in referring individuals for criminal prosecution. He declined to say how many referrals had been made.

Neither Burr, Warner nor the rest of the committee's 13 members participate in the closed-door witness interviews — though they can request a briefing at any time. The day-to-day investigative work and the long-term arc of the inquiry has been driven by the same core team, which has grown slightly from an original staff of seven to nine. Interviews have been as short as one hour and as long as 10 hours.

"They come to agreement between the nine of them on every step they take, every person that they call in; for the most part, every question that they ask is gamed out between all nine of them," Burr said of the team.

The investigation now spans continents and includes sources from countries besides Russia. The staff have traveled overseas and witnesses have come in to testify from abroad. "I don't think we've got any rock that we haven't turned over, regardless of, geographically, where it's located," Burr said.

Though the team is known to work six and sometimes seven days a week, Burr denied it had taken a toll. "Morale's great," he said. "I think that they're fueled by what they find."

And the biggest compliment they receive, he continued, is when witnesses later say they could not tell which staff members were Democrats and which were Republicans. "That's the way it should be," he said.

"It's sort of why I get baffled when I get asked the question, 'Does this fall apart at the end?'"

He paused.

"It's hard for me to believe this could fall apart at the easiest point, which is: 'Here are the facts. Write the report.'"

What's been done so far

The secure spaces where the committee does most of its investigative work are a networked sprawl across the second floor of the Senate Hart building, a stark white structure with a soaring central atrium. As with other secure complexes in the Capitol, it has numerous entries and exits that can facilitate the discreet delivery of a witness.

Inside those spaces, the staff have several investigation boards mapping out known connections between witnesses — and a constellation of players whose roles and relationships are not fully understood.

"There've been a lot of people — a lot of people — that none of you have caught," Burr said, referring to the reporters who linger outside to catch a glimpse of persons of interest.

Smiling as though he had pulled a satisfying prank, he said, "Not all the interviews have taken place up here."

After he and Warner agreed on the investigation's overall parameters, it was initially structured to include three buckets:

a review of the intelligence underpinning an assessment of Russia's efforts to target the 2016 election;

an examination of the "active measures," including cyber activities, that Russia employed; and

an inspection of counterintelligence concerns stemming from possible links between Moscow and the campaigns.

It has since expanded to include at least two additional inquiries: an evaluation of the Obama administration's response to Russia's efforts, and a deep dive on the effect of foreign influence campaigns on social media.

To date, the committee has interviewed more than 200 witnesses and reviewed more than 300,000 pages of documents; it has held more than a dozen public hearings and released two interim reports.

The first, on election security, was issued last March and found that the Department of Homeland Security's response to Russia's incursions was "inadequate." It included a number of policy recommendations about how to better protect U.S. election systems and the voting process.

The second, released in May, included the initial findings of a review of the 2017 Intelligence Community Assessment (ICA) on Russia's active measures — an unclassified version of a more comprehensive report is still forthcoming.

"We see no reason to dispute the conclusions," of the ICA, Burr said at the time, in a simple assertion that nevertheless generated headlines for its contrast with a finding by the House Intelligence Committee's Republican majority.

Their final report cited "significant intelligence tradecraft failings" in the assessment made of Russian president Vladimir Putin's intentions. On a bipartisan basis, the Senate committee substantiated the finding that Putin had developed a "clear preference" for Donald Trump.

Since those reports were released, both Burr and Warner have made overly optimistic statements about the timing of subsequent findings.

That optimism seems to persist. Burr said the committee is "close to pushing out the door" an assessment of the Obama administration's response, estimating that it would be a "matter of weeks." (He had predicted in August that this report, and a second on the role of social media, would be released in September.)

He and Warner have also offered inexact estimates of when the overall investigation would wind down; it was already supposed to have ended by the end of 2017, by spring 2018, before the midterms, and then by the end of last year.

Burr insisted each prediction had come with a disclaimer: "We don't know what we don't know."

What's taking so long?

Burr had not wanted to expand the universe of witnesses the committee interviewed, but felt he had little choice. He guessed that the committee had completed interviews with its target list "fairly early on" in 2017.

"It's not the people that were on the deck that we knew about that lengthened the time. It was the people that we didn't know about that we came to the conclusion — either for the campaign interactions or for this bigger picture that we're looking at — that extended the timeline," he said.

Adding a new witness to the roster added at least three weeks to the investigation, he said, between scheduling, logistical preparations, and dealing with legal counsel.

Though most witnesses were encouraged to come in voluntarily, Burr acknowledged he had resorted to issuing subpoenas — either because all other appeals had gone unanswered, or because witnesses themselves had requested it.

Requested to be subpoenaed?

"Can't get into who," Burr said, "but there are some that" — he intimated an elbow nudge as he imitated a witness saying, "'I'll come if you subpoena me.'"

"They wanted the cover of being compelled," he said.

Burr also argued that a subpoena was not a surefire way for him to get answers since he was effectively out of tools if witnesses, like Trump advisor Roger Stone, indicated they would plead the Fifth.

"They can refuse a subpoena and we can go through a fairly lengthy Senate process to hold them in contempt," he said, "but understand that when I start that process, we've pretty much given up on the ability to have an interview."

"So it's only to be able to say, in the annals of history, 'We used every tool that we could but we couldn't get them here," he said.

One key witness whom the committee had been unsuccessful in engaging, Burr said, was Christopher Steele, the British former intelligence officer who authored the controversial, partially verified dossier, which described links between Trump associates and Russia and played a part in triggering the FBI's counterintelligence investigation.

Last February, the former chairman of the Senate Judiciary Committee, Iowa Republican Chuck Grassley, sent a letter to a Washington-based lawyer acting as an intermediary for Steele asking whether Steele may have been indirectly on the payroll of Oleg Deripaska, a Russian oligarch with close ties to Putin. The implicit suggestion of Grassley's inquiry was that the dossier contained purposeful misinformation intended to help Russia. It is not a view, or a suspicion, that Democrats share.

Burr would only say that Steele remained of interest, but out of reach.

"We've made multiple attempts," to elicit a response, Burr said, but declined to surmise why Steele would not engage.

"You'd have to ask him," he said, referring to Steele. "I think there will always be some questions as to…" he stopped, and paused for a long time.

"…whether his connections to this extend far outside of the contract."

Preexisting connections to what or whom?

"Oh, I can't get into it," Burr said. "Those are things that I'd love to know the answers to, but I don't have the ability to do it. And I'm sure at some point — maybe — we'll know the answer to that."

Burr has previously said it would be impossible to assess the credibility of the dossier without understanding who Steele's sources and sub-sources were; failing to speak directly with Steele suggests that the committee has not, itself, come to a determination of the dossier's reliability.

"I think it's safe to say that we have followed every potential lead and we know a heck of a lot more today than we did two years ago," he said. "But I can't tell you we —" he trailed off again.

"— we know the motivation."

A request for comment from Steele was not returned.

Was there collusion?

It is not lost on Burr, who has been on the Senate intelligence committee for over a decade, that he now leads the same body that has been charged with conducting authoritative investigations of some of the most momentous events — and biggest intelligence surprises — in U.S. history.

The committee has issued unflinching reports on the Iran-Contra affair, Iraqi WMD intelligence, and the intelligence community's activities before 9/11.

It is, of course, the same committee that in more recent years fell apart along partisan lines while investigating the CIA's post-9/11 interrogation and detention programs.

Both Burr and Warner have said they intend to ensure their investigation withstands the test of time.

"I've been around this town long enough to know that little, if any, of the facts surrounding the '16 election — even if we didn't find them — at some point, they're going to be public because somebody's going to write a book that was involved. And they're going to produce documents that maybe we didn't get," Burr said.

"But I look at that with the belief that it will validate what we put in our report, as long as we don't let politics drive what the final conclusion is," he said.

That was the very thing that seemed to derail the ultimately rancorous investigation led in the House by Republican Chairman Devin Nunes of California. The committee's Republican majority, over the protestations of Democrats, issued a final report last spring that found "no evidence" that the Trump campaign colluded or conspired with the Russian government — while acknowledging there had been instances of "poor judgment" demonstrated by some Trump officials.

Democrats, led by Ranking Member Adam Schiff, have vowed to keep the investigation open and issue their own conclusions.

President Trump seized on the Republicans' findings and suggested Nunes should receive the Medal of Freedom while decrying the Democrats' efforts as "presidential harassment" and Schiff as a "political hack."

Comparatively, Mr. Trump has had little to say about the Senate's investigation or its leadership.

"When you don't do something in public, you don't become the target of criticism or praise. And that's fine with me," Burr said.

He and the vice chairman are known to have made an agreement that the committee would not do anything that was not agreed upon by both sides, whether that meant calling in witnesses, issuing subpoenas or opening new — or closing existing — avenues of inquiry.

"I don't know that we've had any big disagreements," Burr said of Warner. "We both committed at the beginning that the investigation would go wherever the facts told us to go."

Still, over time, rumors of a growing friction between the two began percolating on Capitol Hill. They suggested that, for as much discipline as its leadership had demonstrated, the committee was fated to reach an impasse on the same question that had divided other inquiries: was there collusion?

For now, Burr appears to have arrived at his answer. "If we write a report based upon the facts that we have, then we don't have anything that would suggest there was collusion by the Trump campaign and Russia," he said.

The finality of Burr's assertion was jarring — but he had said a version of it before. He told Fox News in September that the committee had found no "hard evidence" of collusion, though new information could still come to light.

He now doubled down, adding it was "accurate with everything we've accumulated since then."

It was the first time the chairman sounded like he was not speaking for the entirety of his committee, given the disconnect between his view of a set of facts and that of the vice chairman. (Warner declined to be interviewed for this article.)

In January, Warner said the revelation that former Trump campaign chairman Paul Manafort shared polling data with Konstantin Kilimnik, a business associate of Manafort's known to have ties to Russian intelligence, was the "closest we've seen" to collusion.

Warner tweeted, "My question is, what did the President know about Mr. Manafort's collusion with Russian intelligence, and when did he know it?"

Burr did not use the word then and would not now. Manafort, he said, "shared polling data with a former partner of an effort to do campaign services in the Ukraine." It was a "stretch," Burr said, to call that collusion.

He was no more persuaded by the broader set of reported interactions between Mr. Trump, more than a dozen of his associates, and Russians.

"I'm unpersuaded because — the majority of contacts — we've talked to, or we've gotten documents from," he said.

He argued that the underlying motivations behind some interactions were often hard, and sometimes impossible, to determine, and that what might look like collusion could have an alternative rationale.

"There's an awful lot of connections of all these people," he said. "They may not be connections that are tied to 2016 elections in the United States, but just the sheer fact that they have a relationship — it may be business. It may be Russian intelligence. It may be they're all on the payroll of Oleg Deripaska," he said.

"We've got to try to determine, in our particular case, 'Do they fit in this bucket' — which is the 2016 election efforts — or 'Do they fit in this bucket,' which is the world that we discovered and that we want to continue to look at on more of a counterintelligence platform," he said.

"In a lot of cases, we found out they fit in neither bucket, or we don't know which bucket. And, in some cases, we've come to the conclusion we will never know the answer; therefore, this question is pushed aside," he said.

In the end, he said, neither his nor the committee's interpretation of the facts should be paramount.

"I have no belief that at the end of our process, people that love Donald Trump are going to applaud what we do. And I have no belief that people that hate Donald Trump are going to reverse and say, 'Well, you know, this clears him.' They are solidly in one camp or the other," he said.

"I'm speaking to what I hope is the 60 percent in the middle that are saying, 'Give me the facts that I need to make a determination in this one particular instance — what happened.' And that's what our focus is," he said.

He would not concede that the committee was destined for disagreement, though he said he had always allowed for the possibility it could happen.

"If the committee's driven based upon the facts that we have at hand, I have a very difficult time understanding how you can come to two different conclusions," he said. "Unless, for the first time, you let politics come into play."

Catching himself slightly, he added, "Now, we're in Washington and so — anything can happen."

What comes next?

Burr has often voiced his awareness that his committee's report will be tested by the special counsel's findings. He has said he's comforted by it, in part because Mueller, by virtue of having more and better investigative tools, may provide answers that proved elusive to his team.

Still, Burr said, "none" of his investigation's timeline is dictated by that of the special counsel; he denied the committee was waiting for Mueller to show his cards before it showed its own.

"If I can finish tomorrow, I would finish tomorrow," he said. "We know we're getting to the bottom of the barrel because there're not new questions that we're searching for answers to."

He added the disclaimer that if a new person of interest arose, the committee would pull the necessary threads.

But he remained evasive as to whether Mueller's final report should itself be made public — even if it could conceivably fill in some gaps within his own probe. "I'm going to leave that up to whoever the A.G. is at the time," he said.

Would he ask — or, if needed, compel — Mueller to testify before the committee, after his work was done?

"Open question," he said. "I don't know the answer."

How the committee will issue its overall findings, once it arrives at them, also appears to be an open question. Burr said a formal draft had not yet been started, and he could not make a prediction about how much of it, ultimately, would be declassified.

He did not say whether the final product would be something like a thoroughly sourced chronology or whether it would include an evaluative judgment — from the investigators who spent two years examining it — on the question of collusion. The latter sounded unlikely.

"What I'm telling you is that I'm going to present, as best we can, the facts to you and to the American people. And you'll have to draw your own conclusion as to whether you think that, by whatever definition, that's collusion," he said.

His last words were of caution.

"My only advice to you is, be careful. There are a lot of false narratives out there," he said.

He said the committee had wasted some of its time interviewing witnesses who, it turned out, just wanted to be part of the story. "Don't get ahead of this process," he said.

"People that are sitting, writing the headlines for our report or for Mueller's report may find out that the headline is significantly different."

"And I think the most difficult thing that we've had to do is to separate fact from fiction through the whole process, because if we took every story that you" — meaning the media — "wrote, or every rumor that's out there, we would never finish."

He laughed a final time.

"There are too many Russians."