Russia’s use of Facebook, Twitter, and other social media platforms to spread fake news and propaganda during the 2016 election has become a “red-hot” focus of FBI special counsel Robert Mueller’s investigation into Russia’s election interference, Bloomberg reported on Wednesday.

The news comes one week after Facebook shut down roughly 470 “inauthentic” accounts and pages that “were affiliated with one another and likely operated out of Russia.” Facebook said in a statement that the accounts were connected to roughly $100,000 in ad purchases between June 2015 and May 2017.

Congressional investigators want to question Facebook and Twitter representatives, too, and the Senate Intelligence Committee is reportedly weighing whether to hold a public hearing focusing on how Russia used social media to “manipulate” voters.

A Facebook spokesperson declined to comment Wednesday.

Sen. Mark Warner, the ranking Democrat on the Senate Intelligence Committee, told reporters last week that the social media element of Russia’s influence operation – which included the hacks on the Democratic National Committee and Hillary Clinton’s campaign chairman, John Podesta – “opens a whole new arena.”

Warner added that he thought the advertisements the Russians purchased were “just the tip of the iceberg.” Facebook has since confirmed that Russia-linked groups went further than ad buys and memes, and tried to organize anti-immigrant, anti-Clinton rallies in Texas and Idaho.

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Russian “bots” posing as Americans also inundated Twitter during the election, spreading anti-Clinton messages and material that had been hacked from the DNC and Podesta.

A New York Times analysis of the Russian social media fingerprints revealed that “hundreds or thousands” of fake accounts on both Facebook and Twitter contributed to the interference campaign.

Americans “ought to be able to know if there is foreign-sponsored [internet] content coming into their electoral process,” Warner said last week. “That becomes a method of influence exponentially, I would argue, bigger than TV and radio.”

Democratic Rep. Adam Schiff, the ranking member of the House Intelligence Committee, said in a statement that he too was “keenly interested in Russia’s use of social media platforms, both the use of bots and trolls to spread disinformation and propaganda, including through the use of paid online advertising.”

Schiff told MSNBC last week that he also wanted to know how sophisticated the ads were – in terms of their content and targets – to determine whether they had any help from the Trump campaign.

Facebook said in its statement that about 25% of the ads purchased by Russians “were geographically targeted.” Facebook representatives told lawmakers behind closed doors Wednesday that the ad sales had been traced back to a notorious Russian “troll farm,” according to The Washington Post.

Warner said Thursday that while “we know about the hacking, as a former tech guy, what really concerns me is that there were upwards of 1,000 paid internet trolls working out of Russia taking over computers, making botnets, and generating news down to specific areas.”