According to the Wall Street Journal, Special Counsel Robert Mueller's investigation into Russian collusion and election tampering included issuing subpoenas for over a dozen Trump campaign officials.
According to the report, the subpoenas were issued in mid-October and the campaign officials have been providing documents to the investigation having to do with the Russian government.
A lawyer for the president said there was "total cooperation" with the investigation.
What do we know so far about Mueller's investigation?
Leaks from Mueller's investigation have indicated that former Trump campaign manager Paul Manafort was being targeted in the probe - he has since been charged with conspiracy against the United States and money laundering.
Former campaign official George Papadopoulos was also reported to have pleaded guilty to lying to federal officials about his interactions with the Russians.
Mueller appeared to be at the center of the controversy over President Trump's anger at Attorney General Jeff Sessions for recusing himself from the investigations. Trump blames Sessions for the creation of the special counsel, though even his allies have pointed out that it was Trump's own actions that precipitated the naming of Mueller.