Why it matters, from MSNBC’s Ari Melber: “[W]e’re moving away from a political fight, where everyone can see it the way they want, and into … a legal process — where there are rules of evidence, facts are established. … Bob Mueller is known to be a pretty careful prosecutor.”

Be smart: Matt Miller — former Obama Justice Department official, and close Mueller watcher, emails me: “I think it means this will be a rolling investigation. Rather than conduct his entire investigation and then wrap things up with indictments and possibly a report at the end, he is doing it in stages, the way the Justice Department might attack a drug cartel or a mafia family.”