Nooses, long a powerful symbol of bigotry and hatred directed at African-Americans, have been found hanging from a tree outside the Hirshhorn Museum on the National Mall; in a gallery at the National Museum of African American History and Culture; outside an elementary school; and on the campus of American University, where bananas with hateful messages were found hanging from nooses on the same day that the first black woman was set to assume the presidency of the university’s Student Government Association.

“To me, a noose is lynching,” said Taylor Dumpson, the woman who became the university’s student government president. “That’s immediately what comes to my mind, that someone is going to hang you, that someone is going to die. That’s a very chilling thing.”

Nooses have also been found in recent months at a middle school in Florida, at a high school in North Carolina and at a fraternity house at the University of Maryland. Also in Maryland, two 19-year-olds are being prosecuted in the hanging of a noose from a light fixture outside a middle school.

At the same time, members of the Ku Klux Klan — an organization whose history is closely enmeshed with the use of nooses in lynchings — appear to be stepping up their public activities. Robed Klansmen appeared at a gay pride march in Florence, Ala., last month, and the Loyal White Knights of the Ku Klux Klan are planning a rally in Charlottesville, Va., on Saturday.

The noose at the Mint was particularly shocking, Ms. Sapp said, because the Mint is under heavy surveillance given its security concerns; employees know they are being recorded as they work. After a daylong investigation, during which the creator of the noose was kept off the factory floor to protect him from physical retaliation, she said he was placed on administrative leave last Thursday and escorted out of the building.