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 J. Rosenstein named Robert S. Mueller III, a former FBI chief.

This was an essential and reassuring step after a series of alarming developments. The first question for Mr. 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 Trump, illegally tried to interfere with investigations into the alleged meddling and collusion.

An independent inquiry is needed because of statements and actions by Mr. Trump that raised serious concern about executive interference. These include his reported request in January that then-FBI Director James B. Comey swear his loyalty to the president; his reported attempt a month later to persuade Mr. Comey to drop an investigation of Mr. Trump’s first national security adviser, who had to quit after he lied about the nature of his contacts with Russian officials; and his decision last week to fire Mr. Comey. Mr. Trump initially put forward false explanations for that firing but eventually admitted that he was motivated by his displeasure with the FBI’s investigation of alleged Russian interference.

A few notes of caution are in order. A special counsel is essential in this case, and Mr. 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 Mr. 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. His job is not to inform the public or to pass judgment on actions that may have been unwise, inappropriate or unethical — but did not violate the law. That is why this appointment does not let Congress off the hook. The American public needs a full accounting of Russian interference in the 2016 election; of American cooperation in that meddling, if any; and of administration efforts to impede investigations into the meddling and collusion, if they took place. The House and Senate intelligence committees are working on aspects of all that, and those must continue. But a full accounting is likely to emerge only if Congress appoints a special commission like the one that investigated the 9/11 attacks. With the Trump administration having led the way, Congress too should act.