Slavery in the Shadow of the Capitol

Found this link in doing research for my cousin--unfortunately it's from 2001 and the links supporting it seem to be broken:

Foreign travelers accounts from the 1830 and 1840 described the Robey and
Williams slave pens which stood along the Mall in the shadow of the Capitol;
the two were often juxtaposed in artworks, and the presence of slave pens in
the center of the nation's capitol captured the attention of abolitionists.
(Ironically, today the Museum of African Art sits less than a block away from
the former location of the Robey and Williams slave pens.)