The United States plans to sanction Russian oligarchs this week under a law targeting Moscow for meddling in the 2016 US election, sources familiar with the matter said on Wednesday, in potentially the most aggressive move so far against the country's business elite.
A grand jury is the way indictments are issued.
The interview is only the "last step", of course, if the president doesn't say or do anything in that interview that merits extending the investigation any further.
Michael Gerhardt, a constitutional law expert who once served as a federal special counsel, said Mueller's actions are not out of the ordinary.
Mr Trump lawyer Jay Sekulow and his former lawyer John Dowd did not immediately comment.
Mueller's description of the president's status has sparked friction within Trump's inner circle as his advisers have debated his legal standing.
Champions League: Upbeat Real Madrid face Juventus
All of the Madrid squad have travelled to Turin, although defender Nacho could miss out with a thigh injury. Captain Sergio Ramos and midfielder Isco have also trained since missing the game at the weekend.
There also remains a legal question about whether a sitting president can be indicted for a crime. The Post said the President is happy to learn that.
The special counsel tells the president's lawyers that he is a "subject", but not a "target", of the investigation.
Nevertheless, Trump has repeatedly expressed an eagerness to sit down for a voluntary interview to answer Mueller's questions - a move Dowd believed would be a mistake, according to a longtime Trump friend.
The person, who was not authorized to speak publicly about private conversations and spoke on condition of anonymity, told The Associated Press on Wednesday that the president is considered a subject of Mueller's probe - not a target.
Mueller reportedly insisted that he needs to interview Trump before he can finish his investigation, but Trump's lawyers have hesitated to allow Trump to testify, partially based on Trump's history.
"Mueller may believe that he can not now indict Trump no matter how much evidence he has against him", Jens David Ohlin, a professor of law at Cornell, told me.
But experts have noted the risk involved in a testimony from the president, particularly as he has become more direct in his criticism of the investigation. Any interview will be a fraught experience for Trump.