Lafayette Square, where hundreds of protesters were cleared by force Monday night before President Trump’s walk to St. John’s Episcopal Church, was once bordered by “slave pens.” Hundreds of enslaved black people were held captive within sight of the White House.

John W. Franklin, senior manager emeritus at the Smithsonian’s National Museum of African American History and Culture, said he thought about the horrifying history of slavery in D.C. when the National Guard, Secret Service and U.S. Park Police used chemical gas, rubber bullets and batons against those protesting a modern form of brutality: the killing of unarmed people of color by white police officers.

Enslaved people helped build the White House. At least eight of the first 12 presidents brought enslaved people with them to labor at 1600 Pennsylvania Ave., according to the White House Historical Association.

“We know about the construction of D.C., but we don’t know who built it, where the slave markets were? Where the slave quarters were? Did you know the site of National Archives is where the D.C. Central [Slave] Market was located on 7th Street? We know Arena Stage on the Wharf, but do we know that is where all the slave ships came in?” said Franklin, the son of renowned historian John Hope Franklin and the grandson of Buck Colbert Franklin, who survived the 1921 Tulsa Race Massacre, “99 years before Trump cleared Lafayette Square to walk to St. John’s. Enslaved people were kept in slave pens along Independence Avenue.”

Lafayette Square was one of hundreds of sites in the United States where enslaved black people were sold during 250 years of slavery, according to the GSA. The nation’s capital was a major hub for the slave trade.