Native Americans had long inhabited both sides of the Mississippi River. The Mississippian culture rulers organized thousands of workers to construct complex earthwork mounds at what later became St. Louis and East St. Louis. The center of this culture was the urban complex of Cahokia, located to the north of present-day East St. Louis within Collinsville, Illinois. Before the Civil War, settlers reported up to 50 mounds in the area that became East St. Louis, but most were lost to 19th-century development and later roadbuilding.