The number of American tech companies involved in the Russian disinformation campaign prior to the 2016 U.S. presidential election keeps growing. Congressional investigators now plan to question Reddit and Tumblr over new reports that their sites may have helped spread disinformation from Russia. Last year, House and Senate lawmakers grilled the executives of Facebook, Google and Twitter, at a trio of hearings in the fall over the same matter.

At least 21 accounts on Tumblr have been linked to Russian troll farm Internet Research Agency. The agency’s content was also found on Reddit topic pages related to then-candidate Donald Trump and Democratic candidate Hillary Clinton. The content had an apparent aim of driving contentious debate and sowing unrest.

Reddit acknowledged earlier this week that it closed hundreds of suspicious accounts in 2015 and 2016. Reddit’s chief executive, Steve Huffman, wrote in a post on the site that the “vast majority of suspicious accounts we have found in the past months were banned back in 2015-2016 through our enhanced efforts to prevent abuse of the site generally.”

Facebook and Twitter revealed that Russian operatives created thousands of misleading accounts on their websites. During the course of the 2016 election, it is estimated that more than 146 million U.S. users on Facebook and Instagram may have viewed content posted by the Internet Research Agency. Twitter has said at least 1.4 million interacted with IRA content, while Google reported 300,000 visitors to YouTube viewing propaganda videos from the agency. The companies have been notifying users who interacted with the Russian propaganda.

Lawmakers continue to explore the ways in which Russia weaponized social media before and during the election. The Senate Intelligence Committee’s top Democrat, Sen. Mark R. Warner of Virginia, said in a statement, “We have continually seen that the IRA’s sophisticated and extensive use of social media – not only Twitter, Facebook and YouTube, but also other platforms – shows a keen understanding of the power of social media in shaping public discourse.”

Huffman said Reddit is cooperating with the investigation. Reddit has banned all ads that come from Russia and continues to search for and shut down accounts tied to Russian propaganda campaigns.

Reddit implemented new forum rules in October that banned content glorifying or inciting violence. Nevertheless, other kinds of controversial content continue to remain. Reddit has fielded criticism for enabling everything from white supremacist content to conspiracy theories to fake sex videos of celebrities.