WASHINGTON - The D.C. mayor says 13 people have died in the shooting rampage, including the gunman.
Mayor Vincent Gray said at a news conference Monday afternoon that the shooter was among the dead. Officials say the gunman has been identified as 34-year-old Aaron Alexis.
Earlier in the day, authorities had been looking for two other people who may have been involved in the shooting. One of those people has been identified and ruled out as a suspect.
However, Gray says investigators are still searching for a third individual wearing an olive-green, military-style uniform to determine if he was involved.
Gray says a motive has not yet been determined.
President Barack Obama promised to make sure, quote, "whoever carried out this cowardly act is held responsible."
He said the victims were "courageous Americans" who knew about the risks of serving overseas, but wouldn't have expected such "unimaginable violence" at home.
The White House says the president has been receiving frequent briefings about the unfolding situation by senior aides.
--Navy Yard has long, colorful history--
By Michael Collins, SHNS
George Washington set aside the land. Abraham Lincoln visited often. And John Wilkes Booth’s body was identified and examined there.
The Washington Navy Yard, the scene of a mass deadly shooting on Monday, has a long, rich history stretching all the way back to the earliest days of the republic.
About a mile from the U.S. Capitol, the 65-acre, walled-off facility is the nation’s oldest Navy installation and sits on land that George Washington set aside for use by the federal government along the Anacostia River.
In the city’s gritty southeast corner, the Navy Yard is in an urban area that in recent years has undergone a renewal fueled in large part by the construction of a baseball stadium, just a few blocks away. Pricy condos, high-rise offices and hotels are all relative newcomers to the neighborhood, as are the thousands of baseball fans who regularly crowd into the stadium for Washington Nationals games.
On Monday, the ballpark served a more sobering purpose -- as the gathering site for Navy Yard workers evacuated after the shooting and their families. The Monday night ballgame was postponed.
Authorized in 1799, the Navy Yard became the Navy's largest shipbuilding and shiplifting facility, according to the Naval District Washington's web site. It later served as a weapons plant.
The Navy Yard was considered an important defense for the city during the War of 1812. But when the British marched into Washington, commanders decided the yard could not be defended and ordered it burned to prevent its capture by the enemy.
During the Civil War, President Lincoln was a frequent visitor to the Navy Yard, which once again played an important role in the defense of the nation's capital.
When Lincoln was assassinated, the eight conspirators accused of plotting his death were brought to the yard and held on vessels anchored on the Anacostia River before their trials. The body of Lincoln assassin Booth was examined and identified on a ship moored at the yard.
Following the Civil War, the Navy Yard was designated the manufacturing center for all ordnance for the Navy and the scene of many scientific developments.
The gears for the Panama Canal locks were cast at the yard. Navy Yard technicians also have worked there on designs for prosthetic hands and molds for artificial eyes and teeth.
Over the years, the Navy Yard has served as the site for various ceremonial events. The first Japanese diplomatic mission was welcomed to the United States at the yard in 1860. Charles A. Lindbergh returned to the yard after his famous transatlantic flight in 1927.
Today, the Navy Yard is the headquarters for the Naval Sea Systems Command, which builds, buys and maintains ships, submarines and combat systems, and is the workplace for about 3,000 people. It's also the home of the National Museum of the U.S. Navy and several other commands.
The Navy's top admiral, Chief of Naval Operations Jonathan Greenert, lives at the yard, and was safely evacuated after Monday’s shooting.