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 try to sway the 2016 presidential election.

"The issue of collusion is still open," said the Republican committee chairman, North Carolina Sen. Richard Burr, who along with the panel's top Democrat, Virginia Sen. Mark Warner, provided the first major update on a congressional investigation that was launched the same month as President Donald Trump was inaugurated.

"The committee continues to look at all evidence to see if there was any hint of collusion," Burr said, adding that "I am not going to even discuss initial findings, because we haven't any."

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

But the committee has still yet to interview some 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 election official take this very 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 said there is 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 public hearing next month. Facebook said Wednesday that it had accepted invitations from both committees, and Twitter confirmed it would testify before the Senate panel.

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. Burr said the committee "has hit a wall" in its requests to interview Steele as he has not accepted offers to meet with staff or with Burr and Warner personally.

"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.

Burr said he was pleading to Steele directly to talk to the committee.

"My hope is that Mr. Steele will make a decision to meet with either Mark or I, or the committee or both so we can hear his side of it," he told reporters, adding: "I say that to you but I also say it to Mr. Steele."

Burr also detailed some of the threads the investigation has pursued without giving any final conclusions about them. He said the panel had interviewed multiple witnesses about an April 2016 event at the Mayflower Hotel in Washington, where then-Sen. Jeff Sessions — now Trump's Attorney General — and former Russian Ambassador Sergey Kislyak attended a foreign policy speech by Trump.

He said they have also talked to people involved with changes in the Republican party platform last year that were seen as pro-Russia, and investigated memos written by former FBI Director James Comey after his meetings with Trump.

Burr said the panel has 25 more witnesses scheduled this month. He threatened any future witnesses who may balk at the committee's invitations with subpoenas.

"If the committee believes you have something valuable to share, I strongly suggest you come in and speak with us," Burr said.