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D.C. Mayor Vincent C. Gray implored city residents to “let this investigation play out” after a second operative from his 2010 campaign was criminally charged Wednesday with lying to FBI agents investigating whether the mayor’s campaign illegally provided cash to another candidate.

Federal prosecutors charged Howard Brooks, who worked on the Gray campaign’s finances, with lying to authorities on April 6, 2011, by claiming he did not personally give money to “Candidate B,” now known to be Sulaimon Brown.

Mr. Brooks is expected to plead guilty to making a false statement during a U.S. District Court hearing Thursday afternoon.

The charges ratchet up the pressure on Mr. Gray a day after his assistant campaign treasurer, Thomas W. Gore, pleaded guilty to making donations to Mr. Brown in other people’s names and then destroying a notebook with a ledger of payments.

Federal prosecutors accused Gore and Mr. Brooks of recruiting Mr. Brooks‘ family and friends to sign their names on five money orders totaling $660 for Mr. Brown’s campaign.

Both Gore and Mr. Brooks are working with investigators in what has been a yearlong probe of the mayor’s campaign that appears to be far from over.

Mr. Gray deflected questions about the probe during his sole public appearance Wednesday after returning from an international retail convention in Las Vegas.

“As I’ve said for the last three or four months, we’ll let this investigation play out, we’ll see what the results of it are and we’ll go from there,” Mr. Gray said from the rooftop bar of the Reef Restaurant in Adams Morgan.

A throng of reporters had followed Mr. Gray up the back stairs of the restaurant to the press event, which touted a loan program designed to help businesses that are adversely affected by streetscape projects at their doorsteps.

When no one asked questions about the program, Mr. Gray quipped, “I wonder why.”

The mayor said he was disappointed that reporters did not show the same level of interest in the city’s small businesses or his recent Las Vegas trip, which may draw economic development to the District, as they have with the campaign scandal.

“Those are very important issues,” he said.

But, as with much of his term, the mayor’s agenda on Wednesday was overshadowed by the scandal.

The fervor around the probe subsided this year before a raid in March on the home and offices of prolific political donor Jeffrey E. Thompson reignited interest in how Mr. Gray’s 2010 campaign raised money and conducted its affairs.

No charges have been filed against Mr. Thompson. But that development, which caught most observers by surprise, shifted attention for a time away from the allegations of illegal payments by Mr. Brown, whose erratic behavior has caused some people to dismiss him as less than credible.

Mr. Brown, a minor mayoral candidate, has long held that he was paid by the Gray campaign and promised a job in the administration to stay in the race and publicly criticize incumbent Mayor Adrian M. Fenty.

Mr. Brooks has maintained a low public profile during the federal probe and has garnered the moniker of “mystery man.”

Mr. Brooks‘ brother-in-law, P. Leonard Manning, told The Washington Times in October that Mr. Brooks once worked with Bruce Bereano, a powerful Maryland lobbyist and longtime friend of Mr. Gray’s who was at the time a consultant for lottery firm GTECH. The firm denied ever directly engaging with Mr. Brooks.

Mr. Manning, a well-connected businessman who ran the D.C. Lottery for more than 25 years, said that while working in the lottery business Mr. Brooks met Lorraine Green, a close confidante of the mayor’s, who then headed the D.C. Lottery Commission and partnered with GTECH in a failed bid on the city’s lottery contract.

Mr. Brown claimed Ms. Green, a former Amtrak executive, participated in the payment scheme, although she has not been charged with any wrongdoing. She told D.C. Council members investigating Mr. Gray’s hiring practices last year that she had a “close, personal relationship” with Mr. Brooks.

Mr. Brooks declined to testify in front of the D.C. Council committee, citing his Fifth Amendment rights. A WUSA-TV (Channel 9) report in October said Mr. Brooks was cooperating with authorities and wore a wire to record conversations for prosecutors.

In court Tuesday, Gore appeared to confirm that Mr. Brooks had worn a wire to record conversations for prosecutors, though his attorney, Frederick Cooke, claimed reporters must have misheard him.

A source with knowledge of the investigation said Mr. Brooks also might have been charged with making donations under another person’s name had he not cooperated with investigators.

After Mr. Gray won the 2010 election, Mr. Brooks‘ son, Peyton Brooks, was hired for a $110,000-a-year job in the Office of the Deputy Mayor for Planning and Economic Development. The younger Brooks later resigned amid the federal probe.

Howard Brooks‘ attorney, Glenn F. Ivey, declined to comment Wednesday.