WASHINGTON — It was the beginning of the night shift last Wednesday at the United States Mint in Philadelphia, a secure facility that manufactures money, when a white male coin maker strode across the factory floor to the workstation of an African-American colleague. He was carrying a piece of rope.

The rope had an official purpose: to seal coin bags once they were full. But the worker, who operates the machinery used to make coins, instead looped and twisted it into a hangman’s noose, according to Rhonda Sapp, president of the Mint workers’ union. She was soon deluged with calls and text messages from outraged employees.

The episode, which has not previously been reported, was confirmed by a spokeswoman for the Treasury Department with a statement saying that the agency has “absolutely zero tolerance” for such hateful displays and that authorities were investigating.

It is the latest in a series of reports this year involving nooses — especially in the nation’s capital — that point to the return of the hangman’s rope as a potent expression of racial animus.