Washington Navy Yard the 'beating heart of the United States Navy'

Shown here is an aerial view of the Washington Navy Yard. (U.S. Navy)

The Washington Navy Yard, where police were responding to a shooter Monday morning, may not be one of the capital's most well-known facilities. But it serves as a major headquarters for several Navy divisions.

Fox News security analyst K.T. McFarland described it as the "beating heart of the United States Navy."

One of the most prominent facilities, which is where the shooter at one point was reported, is the Naval Sea Systems Command headquarters. This headquarters is the office for about 3,000 people.  The command's job is to "engineer, build, buy and maintain ships" and other systems, according to the Navy. Its annual budget is nearly $30 billion -- accounting for a quarter of Navy spending.

Capt. Chuck Nash, former Navy captain, said the command's principle job is "acquisition" and not military operations.

"It's mostly a civilian organization," he told Fox News.

Details about the shooting are still emerging. Officials say 13 people, including the shooter, have been killed.

The Navy Yard as a whole has been in operation since the early 19th century. It is the Navy's oldest shore establishment, and went from being a shipbuilding center to an ordnance plant. Now it is described as the "ceremonial and administrative center" for the U.S. Navy.

It houses the Chief of Naval Operations -- who is Adm. Jonathan Greenert -- and serves as HQ for the Naval Historical Center.

The Navy Yard also houses the Navy Museum, and serves as headquarters for Naval District Washington.