Washington (CNN) Special counsel Robert Mueller has subpoenaed the former social media adviser for Roger Stone, the consultant's attorney told CNN on Wednesday.

Mueller's team issued subpoenas last week to Jason Sullivan, the former Stone adviser's attorney said. Sullivan worked for Stone's PAC, Committee to Restore America's Greatness, before the election and was hired to help Stone with understanding social media, according to Knut Johnson, Sullivan's attorney.

Mueller's ongoing investigation is probing Russian interference in the 2016 presidential election and any potential links between Trump campaign associates and the Russian government.

Reuters was first to report that Mueller had subpoenaed Sullivan.

As the one-year anniversary of the appointment of Mueller as special counsel in the Russia investigation approaches on Thursday, Trump allies have been calling for a swift end to the investigation.

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