More than half of the United States saw Facebook posts tied to Russian trolls during the 2016 U.S. presidential campaign, while the Kremlin had ties to thousands of accounts on Twitter. And in the eyes of Sen. Mark Warner, who’s leading a probe into the matter, the fear is that these and other tech giants have only scratched the surface in their findings.

A day before the Senate Intelligence Committee is set to grill senior Silicon Valley executives, the panel’s top Democrat, Warner, told Recode that he still isn’t sure if the tech industry has done the fullest accounting of Russia’s disinformation efforts on their platforms. To that end, he pledged to press all three companies during questioning, while urging his colleagues to support new regulations targeting online political ads.

To be clear, the senator said the data already shared by Facebook, Google and Twitter is alarming. All three companies had probed their sales records and other data to track the scope of Russia’s suspected election meddling and they released their findings on Monday, ahead of their scheduled testimony in front of a different Senate committee on Tuesday — the first of three such hearings this week focused on the 2016 election.

Among the new revelations: Stories and other organic posts from Russian-tied Facebook accounts reached perhaps as many as 126 million U.S. users in a two-year window around the 2016 election. Twitter, meanwhile, reported Russian trolls had more than 2,700 accounts on the site, more than the roughly 200 the company initially reported.

Taken together, Warner told Recode, “it tells me that, for a relatively small amount of money, with a dedicated group of hackers who are creating fake accounts, and the ability of them to manipulate a number of bots, you can drive any story to vast numbers of people.”

Come Tuesday, when Facebook, Google and Twitter return to the U.S. Capitol to testify in front of the Senate Intelligence Committee, Warner said he has three major goals.

“Hopefully, the companies are going to come even more fully clean with what happened in 2016,” he began. “Second is, I want to know ... what else they’re going to do to work with us to prevent it from happening in the future.” And most ambitiously, the senator said, he wants to “really get the message out to the public, writ large, how pervasive this problem is and how everyone needs to be a little better consumer of news.”

That includes probing each tech giant on the way it polices its platforms.

With Facebook, for example, the company previously reported that it found 470 accounts with ties to the Internet Research Agency, a known online troll group backed by the Russian government. A lingering question for Warner: Why does that number seem small given that Facebook had flagged many more problematic profiles in places like France? There, it took down 30,000 suspicious accounts before the country’s own presidential election.

While the senator acknowledged they are each different contests with different political environments, he still questioned whether Russian disinformation on Facebook was solely “directed out of this one single troll farm,” known as the IRA. And he said he would challenge Facebook at a hearing tomorrow as to “whether they’ve done a thorough enough scrub.” That includes, for one thing, if Facebook looked “at the digital signatures” of those accounts flagged in France to see if they had any links to profiles in the U.S.

With Google, meanwhile, the concern still appears to be its search algorithm. While he described the company as “more proactive” than its peers, Warner pointed to examples before the election when his staff had searched for terms like “election hacking,” and in the results, “four of the first five stories that came up were Russian” in origin.

“You would have thought it would have been a Fox News or a New York Times [link],” he said.

And with Twitter, Warner highlighted that the company had “dramatically increased the number of accounts” tied to Russian sources. He once more raised the question about bots spreading disinformation on the site, given the fact Twitter doesn’t have a policy in place to authenticate its users in the same way that Facebook does. Previously, the senator lashed the company for failing to conduct an exhaustive search of its platform for signs of Russian interference.

To be sure, Warner did praise each company for making “good faith efforts” in recent weeks to improve some of their practices — including their pledges to be more open about the political ads they permit and the sort of audiences that those ads target. But he said those efforts might not go far enough, as companies like Facebook only are targeting ads about candidates, at least to start.

To that end, he called on the industry to support his measure, called the Honest Ads Act, a bill that some in Silicon Valley have been lobbying against in recent weeks.

“I hope they’ll work with us on this,” Warner said. “I consciously tried to make sure this was the lightest touch possible. It doesn’t even touch the creation of fake accounts, and fake pages, which in many ways is the more pervasive and insidious effort, but figuring out a way to legislate around that, that gets dicier.”

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