Story highlights Nooses have been found on a Smithsonian tree and in African-American museum

Museum's founding director: "It is a stark reminder of why our work is so important"

(CNN) For the second time in a week, a noose was found on Smithsonian grounds -- this time, at the National Museum of African-American History and Culture.

The noose was found Wednesday in the history galleries of the Washington museum, which was opened last year by President Barack Obama.

The discovery "is a painful reminder of the challenges that African-Americans continue to face," Lonnie Bunch, the museum's founding director, said in a statement posted on Twitter.

A statement from our Founding Director Lonnie Bunch on the noose found in our history galleries today. pic.twitter.com/sFWVSaobhV — Smithsonian NMAAHC (@NMAAHC) May 31, 2017

"This was a horrible act, but it is a stark reminder of why our work is so important."

The facility -- part of the Smithsonian Institution -- is the "only national museum devoted exclusively to the documentation of African American life, history, and culture," its website says.

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