Two pre-existing settlements were included in the territory, the port of Georgetown, founded in 1751, and the port city of Alexandria, Virginia, founded in 1749. In 1791 and 1792, a team led by Andrew Ellicott, including Ellicott's brothers Joseph and Benjamin and African American astronomer Benjamin Banneker, whose parents had been enslaved, surveyed the borders of the federal district and placed boundary stones at every mile point; many of these stones are still standing. Both Maryland and Virginia were slave states, and slavery existed in the District from its founding. The building of Washington likely relied in significant part on slave labor, and slave receipts have been found for the White House, Capitol Building, and establishment of Georgetown University. The city became an important slave market and a center of the nation's internal slave trade.