The historic building located at 515 North Washington Street in Alexandria, Virginia, United States, has been used as a cotton mill, prison, factory, office space, and apartments. It was built by the Mount Vernon Manufacturing Company in 1848 in present-day Old Town Alexandria. After the original owners were unsuccessful with their cotton mill business venture, the building was sold in 1855. After a renovation, the new owner was able to operate a successful business, but his success was cut short after the Civil War began. The building was one of many local businesses that the Union Army seized after it entered Alexandria. After briefly serving as a hospital and warehouse, the mill was repurposed and renamed the Washington Street Military Prison, the largest of the five Civil War prisons in the city. Rufus Dudley Pettit was named warden in 1864 and he was responsible for widespread abuse and sadistic behavior towards prisoners.

Alexandria is an independent city in the northern region of the Commonwealth of Virginia, United States. It lies on the western bank of the Potomac River approximately 7 miles (11 km) south of downtown Washington, D.C. Alexandria is the third largest "principal city" of the Washington metropolitan area which is part of the larger Washington-Baltimore combined statistical area.