RECENTLY THE U.S. Attorney’s Office successfully argued that former D.C. Council member Michael A. Brown should serve more than three years in prison for accepting bribes. It announced that a former employee in the District’s tax office pleaded guilty to stealing $114,000. And it unveiled the plea agreement of a former D.C. Council candidate caught up in the sham campaign financing of disgraced businessman Jeffrey E. Thompson.

So federal prosecutors have been busy, and D.C. corruption remains in their sights. Still, it is disconcerting that there has yet to be resolution of the most troubling issue to dog the city: whether D.C. Mayor Vincent C. Gray (D) was criminally complicit in the wrongdoing of his 2010 mayoral campaign. Investigations take time and must be careful, but we urge U.S. Attorney Ronald C. Machen Jr. to show urgency in providing an answer to this central question.

It has been three months since Mr. Machen’s office entered into a plea agreement with Mr. Thompson that is controversial not only because of the relatively light sentence promised to the mastermind of millions of dollars in illegal campaign donations over six years but also because of its timing. Mr. Thompson’s guilty plea, featuring allegations of Mr. Gray’s involvement in an illegal shadow campaign that helped elect him in 2010, came three weeks before Mr. Gray lost his bid for reelection in the April 1 Democratic primary. Mr. Gray adamantly denied the allegations, a view that his private attorney Bob Bennett reiterated to us last week.

Were prosecutors, as Mr. Bennett argues, unfair to the mayor? The proposition that they caused Mr. Gray’s loss is undermined by polls, consistent throughout the primary, that showed the mayor with less than 30 percent of the support of those surveyed. He ended up with 33 percent of the vote; D.C. Council member Muriel Bowser (D-Ward 4) seems to have won not because voters deserted Mr. Gray but because they switched their support from other challengers. In addition, a court opinion unsealed last week makes clear that it was Judge Colleen Kollar-Kotelly, not the U.S. Attorney’s Office, who insisted Mr. Gray be identified by name.

But at this point it’s important to finalize a case that has dragged on for too long, albeit not through any fault by the prosecutors. (Destruction of documents by lower-level culprits played a role, as did a long legal battle over access to other documents.) D.C. residents deserve to know if their mayor — and Mr. Gray, lame duck though he may be, is in office for seven more months — was involved in wrongdoing. If Mr. Gray is, as he says, faultless, he is entitled to have his name cleared or be given a forum to make his case.