NEW YORK (CNNMoney) - Silicon Valley is settling in to get grilled by Washington.
Executives from Facebook, Twitter and Google are testifying before Congress Tuesday afternoon in the first of three hearings this week into how foreign nationals used social media to meddle in the 2016 presidential election.
At the hearing, held by the Senate Judiciary Subcommittee on Crime and Terrorism, the tech companies were pressed on their ability to prevent bad actors from taking advantage of their platforms through ads and regular posts.
"You've got 5 million advertisers, and you're going to tell me you are able to trace the origin of all of those advertisements," Sen. John Kennedy, a Louisiana Republican, asked Colin Stretch, Facebook's general counsel.
Stretch admitted Facebook had limitations on what it could know. "To your question about seeing behind the platform, to understand if there are shell corporations, of course the answer is no," he said. "We cannot see behind the activity."
The heated exchange highlights the difficulties the online companies face in trying to identify and crack down on foreign meddling and misinformation campaigns.
In prepared testimony for the first hearing, which is being held by the Senate Judiciary Subcommittee on Crime and Terrorism, the tech companies revealed the sweeping scale of Russian influence operations on their platforms.
Facebook informed lawmakers that roughly 126 million Americans may have been exposed to content generated on its platform by a Russian government-linked troll farm known as the Internet Research Agency between June 2015 and August 2017.
Twitter disclosed that it has identified 2,752 accounts linked to the Internet Research Agency. It found a total of 36,746 accounts that appeared to be associated with Russia, though not necessarily with the Internet Research Agency, which generated automated, election-related content.
At the hearing, Stretch said Facebook also saw some activity continue after the election with the goal of "fomenting discord about the validity" of President Trump's election.
"The bottom line is these platforms are being used by people who wish us harm and wish to undercut our way of life," Senator Lindsey Graham said in his opening remarks at the hearing Tuesday.
The tech companies condemned the Russian activity and pledged to continue investigating it and cracking down where necessary.
"This type of activity creates not only a bad user experience, but also a distrust for the platform," said Sean Edgett, acting general counsel at Twitter. "So we are committed every single day to getting better at solving this problem."
The hearings and new disclosures cast a harsh spotlight on the immense power of the tech companies at a time when there is renewed interest in greater regulation for the industry.
"I'm very proud that the three companies you are presenting here today are American companies and I think you do enormous good, but your power sometimes scares me," Sen. Kennedy said at the hearing.
This month, a bipartisan group of senators unveiled legislation called the Honest Ads Act to require new disclosures for political ads that appear online on sites like Facebook and Twitter.
Both Facebook and Twitter have preemptively promised greater transparency for political ads, but that may not be enough to appease legislators. Senator Mark Warner, the top-ranking Democrat on the Senate Intelligence Committee, suggested he would press the issue at one of the hearings scheduled for Wednesday.
"How do they plan to work with Congress to make sure this doesn't happen again?" Warner wrote on Twitter Tuesday. "Specifically on legislation like the #HonestAds Act."
But Warner and his colleagues won't be able to raise those concerns with tech CEOs at the hearings. The three companies will be represented by their general counsels at the hearings this week, and not any of their more well-known executives.
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