A congressional investigation into Russia’s role in the 2016 presidential election — and the way in which Kremlin-aligned forces may have pursued their efforts on social media — is already expanding beyond early scrutiny of Facebook.

For Sen. Mark Warner, the top Democrat on his chamber’s intelligence committee, there are fresh fears about the extent to which Russian agents may have tapped bots to amplify their message on other websites like Twitter, he told Recode on Wednesday. To that end, Twitter is set to brief staff of the Senate Intelligence Committee, which is leading the probe, next week.

“Just as the case with Facebook ... I think the Russians actively used Twitter accounts,” Warner explained. “They don’t screen out as much anonymous or fake accounts, but I feel like it’s important to let them come in and make that presentation first.”

Warner is also concerned as to what role, if any, other websites like Reddit may have played in spreading misinformation, his office confirmed, after hearing from security experts about misinformation spread there. And earlier this spring, the Virginia Democrat had briefings from Google, a spokeswoman for the senator confirmed. Even now, the Virginia Democrat told Recode, he remains troubled by the way in which Google displayed search results related to the 2016 election.

“I would point out that, you know, back around the election time, if you were to Google election hacking, most of the stories that came up were Russian-based,” Warner said. “That raises a question to me — why wasn’t Fox or MSNBC higher on that list?”

The flurry of uncomfortable questions for these tech giants presages what might be a tough public grilling before the intelligence committee as soon as next month. Warner said his goal is to hold a hearing in October, and the panel’s leader, Republican Sen. Richard Burr, seems to agree. But Burr has not formally set a date, nor have companies like Facebook, Google and Twitter received official requests to testify.

Asked if the panel might soon demand that the chief executives of tech giants like Facebook appear before Congress, Warner replied to Recode: “TBD.”

The Senate probe is one of two efforts by lawmakers to study suspected Russian interference in the 2016 presidential election. Warner’s counterpart on the House Intelligence Committee, Rep. Adam Schiff, similarly told Recode in an interview this week that he would like to see tech giants — including Facebook — explain themselves to congressional investigators.

“To a frightening degree, we see a growth in the use of malicious bots to try to sway public opinion,” Schiff said. “It’s not confined to Russia, and in some cases it’s very human-driven as well as bot-driven.”

At a hearing, Schiff said, lawmakers would certainly have myriad questions — like “what can [tech] tell us about the after-effects of the advertisements they have identified, the downstream consequences that may have been unleashed by it.”

There’s a “great deal more we need to know,” the Democratic lawmaker continued, “and a great deal more the tech companies themselves need to know.”

Spokespeople for Facebook, Google and Reddit did not immediately respond to requests for comment on Thursday.

Twitter, meanwhile, acknowledged it would be visiting Capitol Hill next week. “Twitter engages with governments around the world on public policy issues of importance and of interest to policymakers,” a spokeswoman said, adding that the company “will continue to strengthen our platform against bots and other forms of manipulation.”

For now, those companies have largely kept quiet about potential Russian interference on their platforms, not least because a more formal federal investigation, led by former FBI Director Robert Mueller, is well under way. Facebook has acknowledged that it found “about 470 inauthentic accounts and Pages in violation of our policies,” which bought roughly 3,000 Facebook ads between June 2015 and May 2017. Those pages were “likely operated out of Russia,” the social giant said at the time.

Despite those disclosures, Warner told Recode that he remains frustrated with Facebook.

“I just wonder, a company that prides itself on data, knowledge of all of us. I find it hard to believe it’s taken this long. Maybe they’ve not put their resources behind this,” Warner said.

“It was reported to me they didn’t even run those same accounts” — those who used credit cards to purchase the 100,000 ads in rubles — to see if they “paid for certain things in dollars in different areas,” Warner charged.

In the meantime, Warner said he’s eyeing early legislation targeting issue-based and candidate-focused online political advertising, hoping to create “some repository where the public can go look at that content.” Right now, those ads — spots on Facebook, as well as disappearing content on apps like Snapchat — aren’t cached anywhere, even though television stations and newspapers have to retain copies for public inspection.

Schiff said he had pressed Zuckerberg directly on Russian interference. During a meeting with the Facebook chief executive last month, Schiff said they “had certainly talked about the issue of what the Russians were doing, as well as the broader issues in terms of how people are getting their information now, whether people are exposed to diversity of information or accurate news.”

Still, the Democratic lawmaker would like tech giants to come to Congress anyway. “I would welcome their coming in to testify, and I hope we can agree on that as a committee,” he said. “But I also think all of the techno companies are going to have to make substantial investments in ferreting out the propagation of fake and false stories, the application of that false information, the expanding use of bots and cyborgs and paid social media trolls, because this problem is going to be with us indefinitely.”

Sign up for the newsletter Recode Daily Email (required) By signing up, you agree to our Privacy Notice and European users agree to the data transfer policy. Subscribe