Welcome back, Mr. Mueller

Washington and the country received a much-needed shot of good news Wednesday evening with the revelation that the Justice Department will appoint a special counsel. Deputy Attorney General Rod Rosenstein named Robert Mueller III, a former FBI chief.

This was an essential and reassuring step after a series of alarming developments. The first question for Mueller will be whether the Russian government meddled in the 2016 presidential election. The second question will be whether anyone in the Trump campaign colluded in the meddling. And the third question will be whether anyone in the administration, up to and including President Donald Trump, illegally tried to interfere with investigations into the alleged meddling and collusion.

A few notes of caution are in order. A special counsel is essential in this case, and Mueller must be prepared to follow the evidence wherever it leads. But there’s always a worry when a prosecutor has only one mission that he will pursue it with excessive zeal. That’s less likely with a special counsel under current law than it was under the old independent-counsel statute, and it’s a trap that Mueller seems unlikely to fall into. But it’s worth keeping in mind.

More salient is the fact that the special counsel’s job is only to look for criminal behavior and, if he finds any, to prosecute the wrongdoers.