Law enforcement officers arrived on scene at Washington Navy Yard within minutes of a reported shooting on
A gunman who died in a mass shooting at a naval installation in Washington DC has been identified as Aaron Alexis, unnamed officials tell US media.
At least 12 people were killed and four wounded in the attack at Washington Navy Yard in the US capital.
Alexis was apparently killed at the scene of the rampage, which began at around 8:20 local time (12:20 GMT).
Police were initially searching for two additional "potential" gunman but one has since been ruled out.
Little has been confirmed about Alexis or how he gained access to the installation.
Police responded to reports of two additional suspects in Monday's shooting, but have since cleared one of them, a white male in a tan short-sleeve military-style outfit.
Eyewitnesses at Washington Navy Yard: Gunman 'aimed
Earlier, authorities said they were also looking for a black male, 40-50 years of age, wearing olive kit and seen carrying "a long gun".
At the White House, Mr Obama said he mourned "yet another mass shooting" and sent his thoughts and prayers to the victims.
Washington Police Chief Cathy Lanier confirmed on Monday afternoon that 12 people were deceased. Washington Mayor Vincent Gray said at the same news conference that four people were wounded in the shooting.
One police officer was shot in the legs.
At least three people, including the shot police officer, were brought to Washington Hospital Center by helicopter with "severe" gunshot wounds, said chief medical officer Janis Orlowski.