Then-D.C. Mayor Vincent C. Gray in December 2014. (Kate Patterson/For The Washington Post)

U.S. ATTORNEY Channing D. Phillips’s decision, announced Wednesday, to end the long-running investigation of corruption in Vincent C. Gray’s successful 2010 campaign for mayor settles the issue of whether Mr. Gray will be prosecuted. It was unfair to the former mayor and unhealthy for the city he once governed that this matter was allowed to fester for as long as it did. But while resolution of this issue is welcome, the government’s action — and its refusal to be more forthcoming — raises a new set of unsettling questions.

Foremost: If prosecutors still believe, as they alleged in a plea agreement reached last year with businessman Jeffrey E. Thompson, that Mr. Gray had knowledge of an illegal $653,000 “shadow campaign” funded by Mr. Thompson and executed by Mr. Gray’s close associates, why wasn’t the former mayor charged so that a jury could decide on his culpability? Or if prosecutors have concluded that Mr. Thompson is not credible, why proceed with a plea agreement that gives the once-prominent businessman no more than six months in prison, rather than the seven years he faced for helping to corrupt not just the 2010 election but also previous campaigns?

Federal prosecutors unfortunately were not available to answer those and other questions. Announcement of the probe’s end without additional charges was made via a news release, a marked contrast to the March 2014 news conference detailing Mr. Thompson’s guilty pleas that, then-U.S. Attorney Ronald C. Machen Jr. said, lifted “the veil over political corruption in the District of Columbia.” Mr. Gray’s bitter supporters believe that event, held just weeks before last year’s Democratic mayoral primary, was responsible for Mr. Gray’s loss to Muriel E. Bowser, later elected mayor in the general election.

The U.S. attorney’s statement trumpeted the overall accomplishments of the office and other federal agencies in uncovering more than $3.3 million in illegal contributions to various campaigns between 2006 and 2011 and guilty pleas from 12 people, including seven connected to Mr. Gray’s 2010 campaign. But the public is owed a fuller explanation about the circumstances that surrounded Mr. Thompson’s plea and the decision not to bring charges against Mr. Gray. The only clue offered (Mr. Phillips declined requests for interviews) is the explanation that the Justice Department policy “provides that no prosecution should be initiated against any person unless the government believes that the person probably will be found guilty beyond a reasonable doubt by a jury . . . and that the jury’s guilty verdict probably will be sustained .”

Adding to the murk is Mr. Phillips’s recent appointment as U.S. attorney, selected over acting U.S. Attorney Vincent H. Cohen Jr., who had been Mr. Machen’s main deputy in conducting the investigation. It appears that dissatisfaction with the Gray campaign probe was a factor in Mr. Cohen being bypassed. The chair of the panel that made recommendations to Del. Eleanor Holmes Norton (D-D.C.), who told the White House that Mr. Phillips was her top choice, had supported Mr. Gray’s reelection and had publicly voiced concerns about the investigation.

Four people who have pleaded guilty to felonies for their part in Mr. Gray’s campaign, including Mr. Thompson, await sentencing. We hope prosecutors explain why, if these defendants have been truthful and cooperative and deserve reduced sentences, additional charges against those who allegedly benefited were not brought. If they do not, let’s hope the sentencing judges demand the answers to which the public is entitled.