A noose has been Wednesday on the floor of an exhibit about racial segregation at the National Museum of African American History and Culture in Washington, DC.
The galley was partially closed for about three hours on Wednesday (local time), and US Park Police were called in to investigate what was described by the museum's director as a "horrible act."
"The noose has long represented a deplorable act of cowardice and depravity - a symbol of extreme violence for African Americans," Director Lonnie Bunch said.
Ms Bunch said museum officials do not know who was responsible and told staff the incident "is a stark reminder why the work you do is so important."
The incident comes less than a week after a noose was found hanging from a tree outside the nearby Hirshhorn Museum, news organisation Smithsonian.com said.
"The Smithsonian family stands together in condemning this act of hatred and intolerance, especially repugnant in a museum that affirms and celebrates the American values of inclusion and diversity," the institution's secretary, David Skorton, told the staff in an internal email. "We will not be intimidated."
In early May, the FBI investigated who hung bananas from nooses on the campus of American University, Washington DC, after the school's first black female student government president took office.
The bananas were hung by black string formed into the shape of nooses, according to photos posted on social media.
One message, "Harambe Bait," referred to the gorilla at the Cincinnati Zoo that was killed last year after a young boy fell into the animal's enclosure.
"I regret this happened, apologise to everyone offended, and state emphatically that this incident does not reflect what American University truly is," Mr Kerwin said in a statement.
The incident was at least the second racially tinged incident involving bananas at the school in a year. In September, white students were accused of leaving a banana at the door of one black woman's dorm room and tossing a banana at another woman.