Three gunmen on Monday went on the rampage in the high-security Washington Navy Yard located a short distance from the White House, killing at least 12 persons and injuring several more in an audacious mass shooting.
One of the gunmen was shot dead and police said they were looking for two others who were believed to be at large.
Police said that at least 12 persons were killed and several injured by the gunmen clad in military-style uniforms.
Washington Police chief Cathy Lanier told a news conference that one of the shooters was dead and authorities were searching for a white man and a black man.
"The big concern for us right now is that we have potentially two other shooters that we have not located at this point," Lanier said.
"We have no information to believe that either of these folks are military personnel, but we do have information that those individuals are wearing military-style uniforms," she said.
ABC News reported a gunman, identified as a 50-year-old Navy employee whose employment status had "recently changed", had died at the scene.
Speaking at the White House, President Barack Obama called for a seamless investigation by federal and local authorities and promised to bring those responsible for the attack to justice.
Hundreds of police and naval personnel surrounded the Naval Sea Systems Command headquarters, where about 3,000 service members, civilians and contractors work. The yard is located less than five kilometres from the White House.
Security at the Capitol was enhanced and several nearby schools and offices were locked down. The shooting reportedly began at about 8:20 am when several shots were heard from inside the yard which maintains warships and submarines for the navy.
Security agents rushed to the scene of firing and a US Park Police helicopter was seen hovering over the yard.
Admiral Jonathan W Greenert, the chief of Naval Operations, and his wife were evacuated to the Pentagon from his residence at the yard soon after the shooting began, officials said.
Two law enforcement officials were among the killed. Officials at MedStar Washington Hospital Center said a police officer was among three persons with gunshot wounds brought to the trauma center by helicopter. The police officer suffered multiple gunshot wounds to his legs, a female civilian was shot in the head and hand, and another woman was being treated for a shoulder wound, said chief medical officer Janis Orlowski. All three were in critical condition but their chances for survival were "very good", she said.
Earlier, police spokesman Chris Kelly described one of the suspects as an adult male, about six feet tall with a bald head and medium complexion, dressed in a black top and black jeans.
"The President has been briefed several times about the unfolding situation at the Washington Navy Yard by Assistant to the President for Homeland Security and Counter-terrorism Lisa Monaco and Deputy Chief of Staff Alyssa Mastromonaco," the White House said.
Officials at the nearby Reagan National Airport briefly stopped planes from taking off or landing because of the shooting.
Patricia Ward, a logistics management specialist from Woodbridge, Virginia, was in a cafeteria on the first floor when the shooting started. "I heard three gunshots, pow, pow, pow, straight in a row," she told The New York Times.
"About three seconds later, there were four more gunshots, and all of the people in the cafeteria were panicking, trying to figure out which way we were going to run out. "The workers in the cafeteria wanted us to stay there, but I just ran, I ran out the side doors," she said.
"The commander of the Military District of Washington is determining the operational and security status of military installations in the National Capital Region.
Pentagon Press Secretary George Little said security has been tightened at military installations in the capital. Navy officials are working closely with law enforcement and emergency management representatives from the FBI and the District of Columbia to secure the scene and begin the investigation, he said.
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