NEW YORK (TIP): Special counsel Robert Mueller’s investigation into Russian attempts to influence the 2016 election and possible connections to the Trump campaign is zeroing in on the spread of fake information on Facebook and Twitter, Bloomberg reports.

Social media is a “red-hot” focus of Mueller’s, U.S. officials familiar with the matter told Bloomberg. Mueller’s office declined to comment, Bloomberg said.

Facebook last week said it found about $100,000 in ad spending connected to fake accounts probably run out of Russia that sought to sow political division in the run-up to the election. That followed an April report by the company that detailed campaigns by “information operators” to sway public opinion.

The vulnerability of social media to political manipulation troubles U.S. officials. According to Bloomberg, federal agencies including the Office of the Director of National Intelligence and the Federal Bureau of Investigation are looking into how to prevent future attempts to manipulate voters, including the 2018 midterm election.

Congress is also concerned and says social media companies must provide more information.

The Senate Intelligence Committee Chairman Richard Burr said Tuesday that it’s “probably more a question of when” than if there will be a hearing with Facebook officials as part of his panel’s probe, according to Bloomberg.

“We’re cooperating with the relevant investigative authorities looking into this subject,” Facebook said in a statement to USA TODAY.

The Silicon Valley company declined to say what information it provided investigators. It has also declined to disclose the ads it discovered, saying doing so would violate its data policy and federal law.

Separately, Facebook said September 27, it would tighten controls over who can advertise on its platform, according to Reuters. The change comes in response to the election ads linked to Russia.