WASHINGTON, DC -- NOVEMBER 18 2013: Ronald Machen makes a point during an interview with a reporter. (Andre Chung/for The Washington Post)

RONALD C. MACHEN Jr. is the longest-serving U.S. Attorney for the District of Columbia in nearly four decades. Mr. Machen, who announced Monday that he is stepping down April 1, has accomplished a lot. He also leaves behind an important piece of unfinished business.

For five years, Mr. Machen has led the country’s largest U.S. Attorney’s Office and one with the unique responsibility of prosecuting local offenses as well as federal crimes. He plans to return to private practice; Principal Assistant U.S. Attorney Vincent H. Cohen Jr. will become acting U.S. Attorney. Attorney General Eric H. Holder Jr. called Mr. Machen a “skilled leader, a devoted public servant, and a forceful champion of justice,” and we don’t disagree. His accomplishments are impressive, including convictions in terrorism-related cases, financial recoveries that netted nearly $2.5 billion for U.S. taxpayers and an initiative to investigate claims of innocence in convictions.

Nowhere was Mr. Machen’s impact more pronounced than in his focus on public corruption. He helped secure convictions of or guilty pleas from more than 160 defendants in cases involving both the federal and D.C. government. He won guilty pleas from three sitting members of the D.C. council and doggedly exposed a criminal conspiracy in the 2010 D.C. mayoral election, thereby helping to reshape local politics for the better.

But the work isn’t over: Left unresolved is the critical issue of former mayor Vincent C. Gray’s involvement. It has been a year since businessman Jeffrey E. Thompson pleaded guilty to masterminding an illegal shadow campaign that helped elect Mr. Gray (D) in 2010. Prosecutors alleged that Mr. Gray had detailed knowledge of the scheme; an indictment seemed imminent. Mr. Gray steadfastly denied any wrongdoing and reportedly rejected a plea agreement.

The lack of any visible developments over the last year has left the city and Mr. Gray hanging as questions have mounted. Were prosecutors right to have accepted Mr. Thompson’s plea just weeks before the primary that saw Mr. Gray defeated in his bid for reelection? What agreement did prosecutors offer to Mr. Gray? Why didn’t they follow up with an indictment? Shouldn’t prosecutors either make a case against Mr. Gray or acknowledge they lack the evidence to do so?

Criminal investigations are bigger than any one prosecutor, and they can’t be made to adhere to any schedule. We would hope, though, that Mr. Machen in his final weeks in office would find a way to bring more clarity to the questions that still haunt this city.