With the resignation and expected guilty plea of former D.C. Council Chairman Kwame Brown, a team of federal prosecutors looking into political corruption now has an undivided focus on Mayor Vincent Gray, subject of a long-running investigation that recently picked up momentum with the convictions of two Gray associates.

U.S. Attorney Ronald Machen has been probing the Gray campaign for nearly 16 months and last month secured felony guilty pleas from two Gray campaign aides.

Machen's investigators have been looking into allegations from minor mayoral candidate Sulaimon Brown that the Gray campaign paid him to remain in the 2010 contest to attack Adrian Fenty, the incumbent and Gray's rival.

Already guilty
Two campaign aides to D.C. Mayor Vincent Gray pleaded guilty last month for their roles in covering up a scheme to help Gray's quest for the District's top job. Thomas Gore, the assistant campaign treasurer, and Howard Brooks, a consultant, both pleaded to federal charges, with Gore acknowledging he destroyed evidence. Sentencing has not been scheduled.

The investigation has broadened to include a review of campaign finance practices in the District and the role of several Gray campaign workers in a bid for the city's lottery contract.

For city officials already wearied by months of scrutiny by federal law enforcement officials, the prospect of further disruption was disheartening, and one they'd like to avoid.

Acting Council Chairwoman Mary Cheh urged any city official -- including Gray -- to leave office if they feel they could be charged with a crime.

"You know if you have committed a criminal act," said Cheh, a law professor. "If they haven't gotten to you, just get out."

Ward 2 Councilman Jack Evans said the recent expansion of the Gray probe suggested city hall could be under a cloud for the foreseeable future.

"With the addition of [campaign donor] Jeff Thompson and the lottery stuff, this will go on for a very long time," Evans said. "Depending on what they find, it could have far-reaching effects."

One lawmaker, though, who requested anonymity to speak candidly, predicted Gray wouldn't face charges.

"In each of the other cases there were paper trails and solid evidence. I just don't see anything except for hiring a crazy guy," the legislator said. "It may jam him up for re-election. It certainly doesn't help his image, but I don't see a big shoe dropping unless there's some direct linkage."

Gray, who has denied wrongdoing, said Thursday that Brown's prosecution didn't heighten his nervousness about the investigation into his campaign.

"I don't know what ... the relationship with Chairman Brown is to me," Gray said. "I don't see any relationship."

The investigations into Brown and Gray are separate, but the charge that prompted Brown's resignation increased chatter among lawmakers that Gray should more fully disclose his knowledge of the campaign's activities.

"If the mayor knows deep down that he committed a crime, he should resign," said a second legislator, who also requested anonymity.

At-large Councilman David Catania, in an interview with WTTG-TV, urged Gray to speak out or quit.