A noose was found on Wednesday inside the(NMAAHC), the Smithsonian said in a statement.
According to CBS affiliate WUSA, the noose was found by museum visitors in the segregation section of the history galleries.
NMAAHC Founding Director Lonnie Bunch released a statement via Twitter late Wednesday saying "the noose has long represented a deplorable act of cowardice and depravity" and that the "incident is a painful reminder of the challenges that African Americans continue to face" and "was a horrible act."
An investigation is ongoing. A spokesperson from NMAAHC didn't immediately respond to a request by CBS News for comment.
Wednesday's discovery is the second time in a week that a noose was found at a Smithsonian museum. U.S. Park Police said a noose was hanging from a tree outside the Hirshhorn Museum, which features contemporary art and culture, in the nation's capital on Friday. Officials said in a statement that it was unclear how long it had been there and that it was found by a Smithsonian police officer, WUSA reports.
D.C. Mayor Muriel Bowser tweeted after that incident.
"It is an unfortunate irony that a sign of intimidation/ignorance would be placed on our National Mall where Americans of all walks of life come to learn more about who we are, celebrate our diversity and leave inspired to improve their lives, communities and country," she wrote.