Mueller's office has interviewed Rod Rosenstein, who has authority over Russiagate

The interview was first reported by the Wall Street Journal, which said that it took place in June or July and that it was about President Trump’s removal of James Comey as the director of the FBI. Special counsel investigators have been probing whether the president might have attempted to obstruct justice leading up to Comey’s firing.

Precisely what investigators have asked Rosenstein, or how key a figure he is in the probe, remains unclear. Rosenstein undeniably played a role in Comey’s firing — authoring a memo highly critical of the FBI director, which the White House used initially to justify the firing.