The U.S. Capitol dome is seen at sunset on Capitol Hill, Friday, Nov. 18, 2016 in Washington. (AP Photo/Alex Brandon)

Lawyers from Facebook, Google and Twitter are testifying on Capitol Hill Tuesday afternoon amid mounting political pressure to fully investigate Russian efforts to influence the 2016 U.S. presidential campaign and reveal publicly what they find.

It is a rare moment in the political spotlight for companies that, despite large lobbying teams in Washington, generally seek to avoid such public and potentially unpredictable confrontations. A growing number of lawmakers, have expressed concern in recent weeks about the Russian online influence campaign and are vowing both to expose what happened and work to prevent a recurrence, through legislation if necessary.

Tuesday’s hearing by a Senate judiciary subcommittee comes a day after the prepared testimonies of Facebook and Twitter revealed that the reach of the Russian-connected misinformation campaign on their platforms was much larger than initially reported.

As many as 126 million Facebook users may have seen content produced and circulated by Russian operatives. Twitter said it had discovered that 2,752 accounts controlled by Russians, and more than 36,000 Russian bots tweeted 1.4 million times during the election. And Google disclosed for the first time that it had found 1,108 videos with 43 hours of content related to the Russian effort on YouTube. It also found $4,700 worth of Russian search and display ads.

The most important unanswered question, outside experts say, is whether the tech companies have evidence that might substantiate allegations that the Russians colluded with Donald Trump’s political campaign, which made Facebook in particular a focus of its election efforts in 2016. Trump and his campaign officials have repeatedly denied allegations of collusion, but questions about the role played by Russia are at the heart of investigations by Capitol Hill and Special Counsel Robert S. Mueller III, whose first round of charges against Trump campaign figures were unsealed Monday.

Tuesday’s hearing offers lawmakers a direct and highly public opportunity to question tech company officials about how their platforms were manipulated, what they did in response, and what they plan to do to thwart similar efforts in the future. None of the companies are sending their top internal security researchers to the hearing, opting instead to send senior company lawyers. Also testifying will be Clinton Watts, a former FBI agent and disinformation expert from the Foreign Policy Research Institute, and Michael S. Smith III, a terrorism analyst.

“We are trying in the Subcommittee to lay out the Kremlin playbook on election interference generally, since this is something that they do in a great number of countries,” said Sen. Sheldon Whitehouse (R.I.), the top Democrat on the Judiciary panel hosting the first of the three hearings, in an interview on Monday. “And we are looking to delve into which elements of the the Kremlin playbook were deployed in the United States specifically.”

Expected to testify at Tuesday’s hearing are Facebook’s general counsel Colin Stretch, Google’s director of law enforcement and information security Richard Salgado and Twitter’s acting general counsel Sean Edgett.

The hearing, as well as Wednesday’s hearings before the Senate and House Intelligence committees, comes amid pushes by Sens. Mark Warner (D-Va.), Amy Klobuchar (D-Minn.) and John McCain (R-Ariz.) to pass new legislation forcing tech companies to disclose information about political ads sold and distributed on their networks. Warner, the top Democrat on the Senate Intelligence Committee, said that colleagues on the committee chose to wait until they heard testimony of the tech companies before they signaled their interest.

The bill, dubbed the “Honest Ads Act,” would require digital platforms with more than 50 million monthly viewers to create a public database of political ads purchased by a person or group who spends more than $500. The public file would include the ad, a description of the targeted audience, the number of views it generated, the date and time it ran, its price and contact information for the purchaser.

But even as lawmakers move to prevent future manipulation, they will use the hearings to probe how foreign actors were able to disseminate propaganda. ”Russia will be the star of the hearings,” said Darrell West, the director of the Brookings Center for Technology Innovation.

Beyond providing the public with a fuller picture of election meddling, experts said the hearings symbolize a broader recognition of the significance massive tech platforms hold in American discourse and politics.

“It’s hard to reconcile the tens of billions of dollars of profit they make with the lack of attention they’ve had with something that could possibly affect our democracy,” said Jason Kint, chief executive of Digital Content Next, a trade organization that represents digital media companies. “The questioning is deeply uncomfortable for them because it gets to the root of their business model, which few people really understand.”

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