A special counsel will be tasked with investigating a president’s retention of classified documents at personal redoubts far from the White House. The wrinkle: The current president is joining the former one in facing prosecutorial scrutiny.

That reality for President Biden came into focus with a statement issued by his own attorney general, Merrick Garland, who appointed Robert Hur — a longtime prosecutor — as special counsel Thursday. Mr. Hur previously served as the United States attorney for Maryland and an official in President Trump’s Department of Justice.