Chairman of the Senate Intelligence Committee Richard Burr (R-NC) and ranking member Mark Warner (D-VA) talk during a hearing about Russian interference in U.S. elections in Washington, U.S., June 21, 2017. Credit: Joshua Roberts | Reuters

WASHINGTON — Leaders of the Senate intelligence committee said Wednesday that they have not determined roughly nine months into their investigation whether Russia coordinated with the Trump campaign to sway the 2016 presidential election.

“The issue of collusion is still open,” said the Republican committee chairman, Sen. Richard Burr, who along with the panel’s top Democrat, Sen. Mark Warner, provided an update on a congressional investigation that was launched the same month as President Donald Trump was inaugurated.

More than 100 witnesses have been interviewed — including former Trump campaign chairman Paul Manafort and Trump son-in-law Jared Kushner — and more than 100,000 pages of documents have been reviewed, Burr said.

But the committee has still yet to interview many witnesses related to the Trump campaign and a June 2016 meeting that Kushner, Manafort and the president’s son, Donald Trump Jr., held with Russians. The committee wants to interview Trump Jr. and everyone else involved with the meeting.

The lawmakers said that though they have reached no conclusion about whether the campaign colluded with the Kremlin — the question also at the heart of a separate criminal investigation by special counsel Robert Mueller — their investigation has left no doubt about a multi-pronged Russian effort to meddle in American politics.

“The Russian intelligence service is determined, clever and I recommend every campaign and every elected official take this seriously,” Burr said.

The news conference Wednesday was an effort by the committee to lay out some of what’s been found so far as the 2018 midterm elections approach. Burr said that the committee would ideally finish the investigation before congressional primaries begin next spring, but said he couldn’t put a firm deadline on the probe because they are always finding new lines of inquiry.

Warner later added that there was a “large consensus” that Russians had hacked into political files and strategically released them with the goal of influencing the election. He said Russian hackers had also tested the vulnerabilities of election systems in 21 states, though there’s no evidence that any voting tallies were altered.

The Senate panel has also been focused on Russian efforts to push out social media messages on Twitter and Facebook, and is examining more than 3,000 Russia-linked ads that Facebook turned over to Congress this week. Burr said he does not intend to release those ads.

Facebook has said the ads focused on divisive social and political messages, including LGBT issues, immigration and gun rights and were seen by an estimated 10 million people before and after the 2016 election.

The Senate panel, along with the House intelligence committee, has invited Facebook, Twitter and Google to testify at a hearing next month. Facebook said Wednesday that it had accepted invitations from both committees.

One witness the lawmakers say they’ve been unable to question is Christopher Steele, a former British spy believed to have compiled a dossier of allegations about Trump connections to Russia. The committee “has hit a wall” in its requests to interview Steele as he has not accepted any of the offers.

“The committee cannot really decide the credibility of the dossier without understanding things like who paid for it, who are your sources and subsources?” Burr said.