A noose, a symbol of racial lynching, has been found on the floor of an exhibit about segregation at the National Museum of African American History and Culture in Washington, DC, Smithsonian Institution officials said.

A gallery at the museum on the National Mall was partially closed for about three hours, 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 in an email to museum staff sent to Reuters by a museum spokeswoman.

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.”

A US Park Police spokeswoman confirmed the agency was investigating but declined to provide any further details.

The incident comes less than a week after a noose was found hanging from a tree outside the nearby Hirshhorn Museum, Smithsonian.com, an affiliated news organisation, said.

Speaking at a dedication ceremony in September 2016 for the $540 million African American museum, then-President Barack Obama said the facility tells the story of black America, and “helps to tell a richer and fuller story of who we are.”

The museum contains about 36,000 items that trace the journey of African Americans from slavery in the 1800s to the fight for civil rights in the 20th century and beyond.

The National Museum of African American History and Culture is part of the Smithsonian, which includes 19 museums, including the Hirshhorn and galleries and the National Zoological Park.

It had 30.2 million visits last year, according to its website.

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“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.”