WASHINGTON — Senate Intelligence Chairman Richard Burr said Wednesday the committee is continuing to investigate potential collusion between the Trump campaign and Russian agents meddling in the U.S. election.

Despite the president's dismissal of possible collusion between his campaign and Russian officials as "a hoax," Burr, R-N.C., and Vice Chairman Mark Warner, D-Va., said the panel is continuing to examine the evidence and has not yet made a conclusion.

"The committee continues to look into all evidence to see if there was any hint of collusion," Burr said at a news conference. He said he and Warner were not going to discuss their initial findings on that topic "because we don't have any."

"We have more work to do as it relates to collusion," Burr said.

Warner warned that Russia's use of social media to push out disinformation to U.S. voters "did not end on Election Day 2016." He said the Russians have employed similar tactics to try to disrupt elections in France, the Netherlands and Germany. And he said they are likely to try it again in the 2018 congressional elections in the U.S.

"We need to be on guard," Warner said.

The committee is scheduled to have a Nov. 1 public hearing with officials from Facebook, Twitter, Google and other social media companies to talk about how Russians exploited those sites in last year's election. Warner said it's important for Americans to know whether an ad they see on social media is coming from the Russians.

Burr and Warner also warned that Russians will continue to try to infiltrate state election systems. The Department of Homeland Security last week alerted officials in 21 states that Russians tried to hack into their election systems in 2016.

"The Russian intelligence service is determined and clever, and I recommend that every campaign and every election office take this very seriously," Burr said.

Burr said the panel had been unable to interview Christopher Steele, the former British spy who compiled a dossier on Donald Trump and his alleged ties to the Russian government, leaving the committee unable to verify its findings.

The dossier, which was published online by Buzzfeed, alleges that Russian operatives obtained potentially compromising personal and financial information about Trump. Moscow has denied having any "compromising materials" on Trump, who dismissed the reports as “fake news.”

"Unfortunately, the committee has hit a wall," Burr said. He said the panel has made attempts to contact Steele in England but that offers to meet with him have been rebuffed.

"The committee cannot really understand the credibility of the dossier without understanding who paid for it (and what the sources were)," Burr said. "My hope is that Mr. Steele will make a decision to meet with me or Mark or both."

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Neither Burr not Warner could say when their committee's investigation will end. So far, their staff has interviewed more than 100 witnesses for a total of more than 250 hours and created more than 4,000 pages of transcripts. The panel's investigators also have read more than 100,000 pages of documents, the senators said.

Burr and Warner said the committee will release a public report of its findings when its investigation is done.

"I know that it feels like it's taking a long time," Warner said. "It is taking a long time."

But he added that what's important is "getting it right and getting all the facts."

The Senate Intelligence Committee is one of three congressional panels investigating Russian interference in the 2016 presidential election and possible collusion between the Trump campaign and Russian officials. The House Intelligence Committee and the Senate Judiciary Committee also are conducting probes.

Special counsel Robert Mueller, the former director of the FBI, is leading a separate criminal investigation with a team of prosecutors.

Burr said the committee has gone as far as it can with its review of the memos of former FBI director James Comey, who was fired by Trump in May as Comey was leading a probe of possible collusion between the Trump campaign and the Russians.

Comey wrote up memos to document a series of controversial private meetings he had with Trump, including one in which Trump allegedly pressed Comey to drop the bureau's investigation into ex-national security adviser Michael Flynn.

Burr said "our investigation with this has reached a logical end." He said it's not closed, but the committee has talked to every relevant person in connection with the memos.

Burr told reporters that questions about Comey's firing "are better answered by the Department of Justice or the special counsel." Mueller is reportedly investigating whether the president's firing of Comey could constitute an attempt to obstruct justice.