FOR THE SECOND time this year, a member of the D.C. Council — this time, its chairman — has resigned his seat after being charged by federal prosecutors with breaking the law. Kwame R. Brown was charged Wednesday with felony bank fraud and stepped down from the council hours later, following in the footsteps of Harry B. Thomas Jr., who left his Ward 5 seat in disgrace in January. But the damage done by Mr. Brown’s alleged misconduct remains, further unsettling a city that must wonder just how far the rot in its government goes.

Mr. Brown, first elected to the council in 2004 and its chairman for the past year and a half, is accused by the U.S. Attorney’s Office of “knowingly and willfully” devising a scheme to defraud Industrial Bank of Washington; specifically, overstating his income by tens of thousands of dollars to obtain a home equity loan and to purchase a boat. Only the sparest of details were provided in the charging document, called a criminal information; more information could be released Friday, when Mr. Brown is expected in court to enter a plea as part of an agreement with prosecutors.

Mr. Brown came under federal scrutiny because of financial irregularities in his 2008 reelection campaign. An audit by the Office of Campaign Finance in 2011 found that the campaign failed to report contributions and expenditures totaling more than $270,000 and that more than $200,000 was passed to a company owned by his brother. How and why the government focused on Mr. Brown’s personal finances — and what came of the money pinpointed by the city auditors — could be revealed when the government lays out its case.

What’s become clear is that U.S. Attorney Ronald C. Machen Jr. is — thankfully — serious about rooting out wrongdoing in city government and holding its officials to strict account. In addition to the cases brought against Mr. Thomas and Mr. Brown, Mr. Machen’s office is examining suspect activity in Mayor Vincent C. Gray’s 2010 campaign. Two of Mr. Gray’s campaign aides have pleaded guilty to taking part in a scheme to pay off another mayoral candidate for campaign attacks. Mr. Gray has denied wrongdoing, but the ever-widening probe has tainted his election and threatens to upend his administration.

“I’m shocked by the news. . . . Never would I have imagined something like this would occur,” said Mr. Gray in a statement about the charges filed against Mr. Brown. Indeed, it’s dismaying that the city, which had seemed to turn the corner on municipal management under former mayors Anthony A. Williams and Adrian M. Fenty, would see two members of its legislative branch charged with felonies and its mayor under a cloud.

It’s critical in the days and months ahead that the District’s elected officials work to stabilize the government. Mr. Brown’s resignation sets the stage for the council to select a replacement chairman from the four at-large members; that person will hold the seat until it can be filled by special election. Of those eligible, Phil Mendelson (D) is the best choice; his career since he was first elected to the D.C. Council in 1998 has been free of ethical taint, and his low-key, hardworking style would help bring the council together during this interim period and instill the public confidence that this badly shaken city so desperately needs.