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WASHINGTON — After interviewing more than 100 witnesses and reviewing a thousand times as many pages of documents, the Senate Intelligence Committee has not ruled out that the Trump campaign colluded with Russia to interfere in the 2016 election and has a lot more probing to do, committee leaders said Wednesday.

"The issue of collusion is still open," the committee's Republican chairman, Richard Burr of North Carolina, told a room full of reporters in the Capitol.

In a noteworthy aside, Burr also suggested that Senate investigators had corroborated some parts of a dossier written by a former British intelligence agent that makes damaging allegations against President Donald Trump and his campaign. Burr did not say which aspects of the dossier the committee may have verified or how much.

The dossier accuses the Trump campaign of conspiring with Russia, and it asserts that Russian intelligence agencies have information about Trump's participation in sexual escapades in Moscow, something Trump has denied.

Burr said the committee had "hit a wall" in its attempts to interview the dossier's main author, Christopher Steele, who lives in London.

Related: Senate Intelligence Heads Showcase Bipartisanship in Russia Probe

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But Burr added, "We have been incredibly enlightened in our ability to rebuild backwards the Steele dossier up to a certain date."

Burr added that the U.S. Intelligence community had little light to shed on the dosser's allegations before June 2016. He didn't explain why, but it has long been known that intelligence agencies around that time began to focus on the issue of potential Russian activity around the election. An FBI investigation began in July 2016.

In the end, Burr said, "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?"

Senate Intelligence Committee Chairman Richard Burr (R-N.C.) (L) and committee Vice Chair Mark Warner (D-Va.) hold a news conference on the status of the committee's inquiry into Russian interference in the 2016 presidential election at the Capitol on Oct. 4, 2017 in Washington. Chip Somodevilla / Getty Images

Burr and the committee's ranking Democrat, Mark Warner of Virginia, stressed that the Russian interference campaign has continued after the election.

Burr said the Russians had been "pretty darn successful" in fomenting chaos in the U.S. political system.

The committee had spent many hours reviewing the assessment by the CIA and other intelligence agencies that Russia interfered in the U.S. election, Burr said, interviewing every person who helped craft it and examining intelligence from "the cutting-room floor" not referred to in the assessment.

The committee generally endorses the intelligence community's conclusions, he said.

Related: Sen. Warner: Trump-Russia Probe Slowed by Trump's Recent Actions

President Trump, however, continues to make statements calling into question whether he believes the same.

On Sept 23, he tweeted: "The Russia hoax continues, now it's ads on Facebook. What about the totally biased and dishonest Media coverage in favor of Crooked Hillary?"

Burr said Senate investigators have interviewed 100 people, with another 25 interviews scheduled. They have reviewed more than 100,000 pages of documents, he added.

The Russia hoax continues, now it's ads on Facebook. What about the totally biased and dishonest Media coverage in favor of Crooked Hillary? — Donald J. Trump (@realDonaldTrump) September 22, 2017

Warner complained that he has seen no evidence that the Trump administration has mounted a government-wide effort to make sure that no foreign power can ever again interfere in an American election. And he said the role of social media in the Russian campaign is only beginning to be explored.

The House Intelligence Committee is conducting its own investigation of Russian election interference, as is the special counsel, Robert Mueller, who is also investigating whether the president sought to obstruct justice.

The Senate Intelligence Committee is trying to complete its work as quickly as possible, the two leaders said, but new strands have continued to arise, including the revelation that the Trump organization was negotiating during the election campaign to build a Trump Tower in Moscow.

Burr said he had a message for witnesses who are reluctant to speak to the committee: "If you don't voluntarily do it, I can assure you today, you will be compelled to do it."