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– Russia’s effort to influence U.S. voters through Facebook and other social media is a 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 is 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.

The ability of foreign nations to use social media to manipulate and influence elections and policy is increasingly seen as the soft underbelly of international espionage, another official said, because it doesn’t involve the theft of state secrets and the U.S. doesn’t have a ready defense to prevent such attacks.

Agencies including the Office of the Director of National Intelligence and the FBI are now examining what could be done to prevent similar interference and espionage in future elections, starting with the 2018 midterm congressional vote, the official said. At the same time, Russia is ramping up its hacking operations, Director of National Intelligence Dan Coats said.

“Russia has clearly assumed an even more aggressive cyber posture by increasing cyber espionage operations and leaking data stolen from those operations,” Coats said Wednesday at the Billington Cybersecurity Summit in Washington.

Russian officials have repeatedly denied their government was behind hacking in the U.S.

The focus of Mueller’s probe comes as the Republican and Democratic leaders of the Senate Intelligence Committee, which is conducting its own investigation, say social media companies including Facebook have to be more forthcoming about what they saw occurring on their platforms last year and how they have responded.

Facebook said last week that it found about $100,000 in ad spending connected to fake accounts probably run from Russia. That followed an April report by the company that outlined coordinated campaigns to misinform the public.

Intelligence Committee Chairman Richard Burr, R-N.C., 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. Mark Warner, the committee’s top Democrat and a former telecommunications company founder, said Facebook’s revelation appears to be “the tip of the iceberg. I think there’s going to be much more.”

Mueller’s investigation is also examining the lobbying activities of Michael G. Flynn, the son of Trump’s former national security adviser, according to people familiar with the probe.

The inquiry into the younger Flynn, first reported by NBC News, follows other indications this week that investigators are increasing pressure on his father, retired Army Lt. Gen. Michael Flynn, who advised Trump during the campaign and briefly served in the White House before being ousted for misleading statements about his contacts with the Russian ambassador.

Mueller is looking at the younger Flynn because of his role as chief of staff to his father at the Flynn Intel Group, a lobbying and consulting firm that worked for international and domestic clients, people familiar with the inquiry said.