Special Counsel Robert Mueller is seeking documents from a witness regarding the president and several of his closest advisers and campaign associates, CBS News confirms, as part of his investigation into Russian meddling in the 2016 presidential campaign.

Axios and NBC first reported the document subpoena, saying that investigators are looking for emails, text messages, work documents, telephone logs and other relevant documents dating back to November 1, 2015. Axios reported that the subpoena was sent to the witness last month.

The document subpoena seeks documents and communications relating to and involving emails to and from the following people:

Carter Page: a former foreign policy adviser to the Trump campaign;



Corey Lewandowski: Former Trump campaign manager;



Donald J. Trump;



Hope Hicks: former White House communications director;



Keith Schiller: former director of Oval Office operations;



Michael Cohen: Personal lawyer for Donald Trump;



Paul Manafort: Former Trump campaign chairman who has been indicted for fraud;



Rick Gates: Former Trump deputy campaign chairman, who pleaded guilty to fraud and lying to federal investigators;



Roger Stone: Informal Trump adviser;



Steve Bannon: Former chief White House strategist



There are some figures known to be key to the investigation into Russian meddling in the 2016 presidential campaign who were not on this list, including former National Security Adviser Michael Flynn and the president's son-in-law, Jared Kushner. The document subpoena is, however, a sign that the investigation is active and ongoing, despite Trump attorney Ty Cobb's repeated predictions over the past several months that the investigation would be wrapping up shortly.

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Recently, special counsel Robert Mueller charged 13 Russian nationals and three Russian companies with interfering in the 2016 election by assuming American identities, sowing national discord on social media, communicating with unwitting Americans and setting up political rallies from afar.

President Trump maintains that there has been "no collusion" on behalf of his campaign. He tweeted on Monday, questioning the timing of the Russia investigation, saying the Obama White House "wanted to discredit" his campaign so Hillary Clinton could win the 2016 election. "Unprecedented. Bigger than Watergate!" he exclaimed.