The United States Justice Department Special Counsel Robert Mueller has issued two subpoenas to a social media expert who worked for longtime Donald Trump adviser Roger Stone during the 2016 presidential election campaign.

The subpoenas were delivered late last week to lawyers representing Jason Sullivan, a social media and Twitter specialist Stone hired to work for an independent political action committee he set up to support Trump, Knut Johnson, a lawyer for Sullivan, told Reuters on Tuesday.

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.

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Stone appeared before the U.S. House of Representatives Intelligence Committee last September and denied allegations of collusion between the president’s associates and Russia during the election.

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

According to sources familiar with the ongoing investigation, Mueller also has been probing whether anyone associated with the Trump campaign may have helped Assange or the Russians time or target the release of hacked emails and other social media promoting Trump or critical of Democratic candidate Hillary Clinton.

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A spokesman for Mueller declined to comment. Russia has denied interfering in the election.