“The committee continues to look into all evidence to see if there was any hint of collusion,” Chairman
The committee, one of four congressional panels investigating Russian interference in last year's election, has reached "a general consensus" that it agrees with the U.S. intelligence assessment from January that indicated Moscow carried out a wide-ranging campaign to influence American voters before they went to the polls, Burr said.
The Senate panel hoped to finish its inquiry and issue findings by the end of December, Burr said. But the probe has expanded into new areas, including Russia's clandestine use of Facebook, Twitter and other social media, leaving the timetable uncertain, he said.
Warner stressed that Russian attempts to meddle in U.S. elections have not stopped, making it important for the committee to complete its investigation before states hold primaries ahead of the 2018 midterm congressional elections.
"You cannot walk away from this and believe that Russia is not currently active in trying to create chaos in our electoral process," Warner said.
Russian intelligence services were "determined" and "clever," Burr said, and state and federal political campaigns and election officials need to treat the threat "very seriously."
Senate investigators have interviewed more than 150 witnesses, including some Trump family members and campaign aides. They also have reviewed 100,000 pages of documents, including emails, phone records, campaign papers and intelligence reports, the lawmakers said.
But the panel also has run into roadblocks.
Christopher Steele, a former British intelligence officer and author of an unverified dossier suggesting that Russian intelligence officials had gathered compromising information about Trump, has refused to talk to the committee, Burr said.
"We have on several occasions made attempts to contact Mr. Steele, to meet with Mr. Steele," he 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," Burr added.
Steele, now a private investigator, investigated Trump while working last year for a Washington opposition research firm, Fusion GPS. The firm's founder, former journalist Glenn Simpson, testified in August before the Senate Judiciary Committee, which also is investigating Russian election interference.
The Senate Intelligence Committee is leaving some areas that it initially had investigated to Special Counsel Robert S. Mueller III, including Trump’s firing of FBI Director
Mueller is conducting a criminal investigation into whether Trump colluded with Russia and other issues, including whether the firing of Comey involved obstruction of justice.
On Monday, Facebook handed over to the Senate and House intelligence committee more than 3,000 ads purchased during the 2016 election campaign by a firm with ties to Russian intelligence.
At least one of the ads, which focused mostly on divisive social issues, such as race, immigration, gay rights and gun control, was seen by an estimated 10 million Americans, the company said.
An estimated 44% of the viewers saw the ads before the November election, and 56% saw them later, the company said.
In some cases, the ads directed viewers to follow fake websites and Facebook pages that appear to have been set up by Russian entities that focused on spreading the divisive issues.
Only 1% of the ads made use of a targeting technique Facebook offers that allows ad buyers to send ads directly to users who went to the fake websites, said a person briefed on the ads who spoke on condition of anonymity.
The ads were geographically targeted, appearing in several key election battleground states, including Florida, Michigan and Wisconsin. They also appeared in Maryland, Texas and Alabama, states that were not closely contested by the Trump or Hillary Clinton campaigns, the person said.
Burr called Russia's use of social media "indiscriminate," noting that in many cases, the Russian ads "didn't even take advantage of targeting."
"The overall theme of the Russia involvement in the U.S. election was to create chaos at every level," he said. "They've been pretty darn successful."
Facebook disclosed the existence of the ads last month. It said they were purchased through 470 fake accounts traced to the Internet Research Agency, a Russian entity known for posting pro-Kremlin propaganda online.
U.S. intelligence agencies said in January that the Internet Research Agency was likely financed by "a close Putin ally with ties to Russian intelligence."
Facebook, Twitter and Google are scheduled to testify at a public Senate Intelligence Committee hearing on Nov. 1.
Warner said the companies are likely to disclose more extensive use of their platforms by Moscow when they testify at the hearing. "I think they've got some more work to do," he said.