If the Federal Bureau of Investigation is to recover its lost reputation, the first item on Attorney General William Barr’s agenda must be to make good on something that set off a frenzy when he proposed it last week: an honest accounting of the FBI’s spying on the presidential campaign of Donald Trump. To do it, Mr. Barr would be well advised to use a grand jury, the same tool used by special counsel Robert Mueller in his investigation.

Right now the American people don’t even know when the FBI investigation into the Trump...