WASHINGTON (Reuters) - A social media expert who worked with influential Donald Trump ally Roger Stone during the 2016 U.S. presidential race testified before a federal grand jury on Friday after being subpoenaed by the special counsel investigating potential collusion between Trump’s campaign and Russia.

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Jason Sullivan arrived at the federal courthouse in Washington at around 9 a.m. and left the courthouse at around 11:30 a.m. After his closed-door grand jury testimony ended, Sullivan declined to comment on what questions Special Counsel Robert Mueller’s team had asked him.

Knut Johnson, one of two lawyers who accompanied Sullivan to the courthouse, said because the testimony was part of an “ongoing investigation,” neither Sullivan nor his lawyers wished to discuss details of it.

Johnson said Sullivan was “open” and “truthful” in his statements to the grand jury. Sullivan received two subpoenas last month from Mueller’s office, one summoning him to testify before the Washington grand jury and the second requiring him to provide documents and other data to investigators, according to copies of the subpoenas seen by Reuters.

Sullivan is a social media and Twitter specialist who worked for an independent political action committee formed by Stone, a long-time Trump adviser and backer, to support Trump’s campaign.

Stone has said Sullivan is one of eight of his associates approached by Mueller’s investigators.

This outreach to Stone’s associates suggests Mueller may be focusing in part on Stone and whether he may have had advance knowledge of material detrimental to Trump’s 2016 Democratic opponent Hillary Clinton that U.S. intelligence agencies have concluded was hacked by Russia and sent to WikiLeaks founder Julian Assange to publish.

“Welcome To The Age of Weaponized Social Media,” said a strategy document Sullivan prepared for Stone and seen by Reuters. Sullivan described a “system” he devised for creating Twitter “swarms” as “an army of sophisticated, hyper-targeted direct tweet automation systems driven by outcomes-based strategies derived from REAL-TIME actionable insights.”

In an emailed statement on Friday, Stone said, “All of the social media activities undertaken by Jason Sullivan were perfectly legal and within the norms of effective political communication.”

“I reiterate that I had no advance notice of the content, source and exact schedule of the Wikileaks DNC disclosures,” Stone added. “I received nothing from Wikileaks or Julian Assange or anyone else including allegedly hacked emails and passed nothing into Donald Trump or the Trump campaign.”

Stone previously has denied having an inside track to WikiLeaks or others who hacked or published Democratic Party and Clinton-related emails that surfaced during the campaign. Stone has said no one from Mueller’s team has tried to contact him.

In September Stone appeared before the House Intelligence committee for almost three hours and denied accusations of collusion between the president’s associates and Russia during the 2016 election.

Trump has denied collusion and called Mueller’s investigation a “witch hunt.”

The Senate Intelligence Committee is also stepping up an investigation into WikiLeaks and how the website obtained and distributed hacked emails during the campaign, said a congressional source, speaking on condition of anonymity.