The gallery containing an exhibit on segregation was closed for about three hours while U.S. Park Police investigated the incident in the nation's capital, Smithsonian officials 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," Smithsonian Secretary David Skorton wrote in an Institution-wide email.
Museum founding Director Lonnie Bunch III said in a statement that the incident is a reminder of the challenges African-Americans still face.
"The noose has long represented a deplorable act of cowardice and depravity - a symbol of extreme violence for African Americans," Bunch said. "This was a horrible act, but a stark reminder of why our work is so important."
Another noose had been found Friday on the grounds of the Hirshhorn Museum, which features contemporary art and culture. Officials said it was unclear how long that noose, which was found by a Smithsonian police officer, had been there.
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