Prosecutors have already questioned at least two dozen current and former Trump White House aides plus several more who worked on the 2016 presidential campaign. Mueller also has access to myriad pieces of evidence surrounding the president thanks to wiretaps, congressional testimony and documents submitted both voluntarily and through subpoena.

Also helpful to Mueller as he investigates: Trump’s own comments about the Russia probe gleaned from media reports, the president’s public interviews and his tweets.

Mueller still has reasons for wanting a Trump interview, namely to give the president an opportunity to explain himself in his own words about his reasons when firing his former FBI director James Comey. That information could go a long way for the special counsel as he weighs making such landmark decisions as whether to indict a sitting president or when writing a final report that could be used to launch impeachment proceedings in Congress.