Representatives from Facebook, Twitter and Google will testify before a series of congressional panels this week about how Russia may have used the social media platforms to influence the 2016 presidential election.

Colin Stretch and Sean Edgett, the general counsels for Facebook and Twitter, and Richard Salgado, Google’s director of law enforcement and information security, made their first appearances Tuesday in a Senate Judiciary subcommittee hearing about Russia’s attempts to spread disinformation online, along with how tech companies can help fight “fake news” and other extremist content in the future.

Read the latest from the hearing here.

Throughout the week, the representatives are expected to face questions about the ways hackers and other influencers associated with Russia may have disseminated fake news, created false accounts and otherwise used online platforms in an effort to manipulate the race between President Donald Trump and former Secretary of State Hillary Clinton — including what the tech giants knew about infiltrations of their systems, and when.

The subcommittee will also hear Tuesday from terrorism analyst Michael S. Smith and Clint Watts of the foreign policy research institute.

READ MORE: A lot happened in the Russia investigation today. Here’s what you need to know

The Washington Post reported late Monday that Google planned to tell senators that Russian trolls “uploaded over a thousand videos to YouTube on 18 different channels.” The Post and Bloomberg also reported, based on a review of draft testimony from the companies, that content produced by Russian operatives may have reached as many as 126 million Facebook users, though company executives said many of those users were not likely to see the content.

In the months leading up to this week’s hearings, the companies have revealed more about how Russian-linked companies used their platforms. Facebook reported earlier this month that 10 million users were exposed to 3,000 ads purchased by a Russian troll farm during the presidential campaigns.

Sen. Lindsey Graham, R-S.C., told Politico last week that he would likely raise proposed legislation that “would mandate new transparency for online political advertising” during Tuesday’s hearing.

Facebook and Twitter have both said they will be more transparent about political ads on their platform, including providing more information about who is funding them.