The Yard is home to the Navy's Sea Systems Command (NAVSEA) - the largest of all the Navy's system commands, with a budget of $30 billion. The command is responsible for designing the concepts behind and engineering of modern ships, their weapons and their operating systems.
The 3,000 workers in NAVSEA's headquarters building - the site of the shooting - occupy just part of the 2.2 million square foot of office space in the Yard, which is situated just north of the Anacostia River in D.C.
The Yard in total is home to about 16,000 employees, all of whom work in facilities converted from ordnance factories in the decades immediately following the conclusion of World War II.
Opened in 1799, served American Naval interests in every war since the birth of the nation. It and the neighboring Marine Barracks 8th and I fell to the British in the War of 1812, and served as Lincoln's key Naval headquarters during the Civil War.
It was the largest ordnance producer in the world during WWII.
Because modern ships eventually no longer fit in the Anacostia River, the Yard and its office space had been converted for several different functions. Along with NAVSEA, it's home to the United States Navy Band, an official Navy Historical Museum aboard the retired USS Barry, the Chief of Naval Operations, the Naval Historical Center, the Marine Corps Institute, and a few other administrative buildings.
It also, as one could imagine, houses a number of higher classified Naval personnel and their operations.