(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.

"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.

The museum contains more than 36,000 artifacts and features exhibitions on African-American history and contributions to the civil rights movement, the military, business, politics, sports and the arts.

Asked about the discovery of a noose, US Park Police said it was investigating "a case of that nature" but did not provide further details.

On Friday, a Smithsonian police officer reportedly found a noose hanging from a tree while making security checks on the grounds.

Nooses are often associated with the historic lynchings of African-Americans, and public displays of nooses are typically viewed as racist acts.