From the Library of Congress website: "Slavery in the District of Columbia ended on April 16, 1862, when President Lincoln signed a law that provided for compensation to slave owners. An Emancipation Claims Commission hired a Baltimore slave trader to assess the value of each freed slave, and awarded compensation for 2,989 slaves."
http://www.loc.gov/exhibits/treasures/trm009.html
And Maryland was considered a slave state until Lincoln arrested the Maryland legislature and stripped them of their Constitutional rights.