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So far, the Gray investigation has resulted in guilty pleas from three aides to the campaign with several others implicated but not indicted in the scheme. Neither Mr. Thompson nor Mr. Gray has been charged with a crime.

With the status of that investigation unclear, Mr. Gray has moved forward with his political agenda and has yet to say whether he plans to seek another term in office. But any progress in city public corruption investigations returns attention to the uncertainty about the mayor and creates a fresh public disruption as candidates begin to announce their intention to seek his job next year.

Mr. Machen noted that Brown embarked on his scheme even after his former colleagues pleaded guilty to public corruption charges, and he reiterated his commitment to pursue such investigations.

“If you doubt our resolve, don’t,” he said. “If you believe you’re too smart or your scheme is too complex, you are wrong.”

• Celina Durgin contributed to this report.