ATLANTA — The brick, marked with a threatening message, crashed through the window of a prominent contractor’s dining room here in September 2015, apparently sometime between dusk and dawn. For some time, news of the incident failed to reverberate much beyond the home itself.

The same went for the dead rodents that had been simultaneously placed on the doorstep of the contractor, Elvin R. Mitchell Jr., and the message: “ER, keep your mouth shut!!! Shut up.”

But in recent weeks, the brick, the rodents and the threat have become troubling symbols of a widening federal bribery and corruption investigation revolving around the granting of city contracts. The inquiry has already resulted in Mr. Mitchell’s and a second contractor’s guilty pleas to federal bribery charges, and it is spreading unease through the civic culture of Atlanta. Municipal contracting here has served a historically important role in the effort to spread wealth to minority businesses, but it has also, at times, been a source of explosive scandal.

On Tuesday, more tremors rippled through City Hall as Jenna Garland, a spokeswoman for Mayor Kasim Reed, said in an email that city’s chief procurement officer, Adam Smith, had “been relieved of his duties effective immediately.” Federal investigators also removed some items from Mr. Smith’s office on Tuesday. Justice Department representatives declined to comment on the matter.