Russia’s effort to influence U.S. voters through Facebook and other social media is a “red-hot” focus of special counsel Robert Mueller’s investigation into the 2016 election and possible links to President Donald Trump’s associates, according to U.S. officials familiar with the matter.

Mueller’s team of prosecutors and FBI agents is zeroing in on how Russia spread fake and damaging information through social media and are seeking additional evidence from companies like Facebook and Twitter about what happened on their networks, said one of the officials, who asked not to be identified discussing the ongoing investigation.