It was an ordinary Friday at City Hall, and Mayor Eric Adams was at the microphone, flanked by two men who each commanded his own vast corner of New York City government.

One was the deputy mayor for public safety, Philip B. Banks III, who had wide influence over the city’s police and fire agencies. The other was his brother, the schools chancellor, David C. Banks, the leader of the largest school system in the United States.

The brothers bantered back and forth about the upcoming school year in a scene that underscored the power that, under Mr. Adams, had been concentrated in the hands of a single family.

Within a year, that air of poise and confidence would be shattered. The administration the brothers served would be engulfed in at least four separate federal corruption inquiries. Agents would show up at their doors, and at the door of their youngest brother, Terence Banks, and seize their phones as part of a bribery investigation. David Banks would announce his resignation as schools chancellor, and Mr. Adams would be indicted on corruption charges.

The inquiry involving the Banks brothers has focused at least in part on government contracts and a consulting firm run by Terence Banks, and its full scope remains unclear.

The brothers have denied any wrongdoing. David Banks said in a statement that federal prosecutors had told his lawyer that he was not a target of the investigation. A lawyer for Philip Banks predicted that his client would be cleared of any misconduct and charged with no crimes. A lawyer for Terence Banks said that the federal inquiry was a bribery investigation, but no bribery had occurred.