(REUTERS) — U.S. 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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