Robert Mueller has served grand jury subpoenas to a social media consultant with ties to longtime Trump ally Roger Stone, in yet another sign that the Special Counsel investigation of Russian election interference may be far from over.

Lawyers representing Jason Sullivan — who describes himself as “Chief Social Media Strategist directly to Roger J Stone Jr.” during the 2016 presidential election — were served with the subpoenas late last week. Sullivan reportedly performed services for an independent political action committee to back President Trump’s campaign.

Sullivan operates Cyphoon.com, a social media company that offers clients the ability to use “an army of sophisticated, hyper-targeted direct tweet automation systems driven by outcomes-based strategies derived from REAL-TIME actionable insights.”

“The subpoenas suggest that Mueller, who is probing Russian meddling in the 2016 U.S. presidential election, is focusing in part on Stone and whether he might have had advance knowledge of material allegedly hacked by Russian intelligence and sent to WikiLeaks founder Julian Assange, who published it,” Reuters reports.

Testifying in front of the House Intelligence Committee last fall, Stone denied claims that the Trump campaign colluded with the Russians.

“I am aware of no evidence whatsoever of collusion by the Russian state or anyone in the Trump campaign,” Stone told Congressional investigators.

Mueller has requested Sullivan appear before a grand jury on May 18th at the Federal Courthouse in Washington, D.C. The social media expert was also subpoenaed to hand over documents related to work performed for Stone.