The Navy said at least three shots were fired at 8:20 am (13:20 GMT) in the headquarters building of the Naval Sea Systems Command.
About 3,000 people work at the headquarters.
Al Jazeera's Rosiland Jordan, reporting from Washington, said the shooter may have barricaded himself on Navy Yard grounds. Witnesses saw police with rifles running towards the building where they believed the attacker might be.
One of the injured was said to be a police officer, shot in both legs.
Twelve people were shot this morning. And two of them were police officers on duty. And just moments ago, four were announced dead.
There's also a local angle to this story. About an hour ago, Tim Hogan (who works communications for Rep. Steven Horsford [D-North Las Vegas]) tweeted photos from the crime scene. Another Horsford staffer, Don Andres, was near the Navy Yard at the time of the shooting.
The shooter(s) apparently used an AR-15 military grade assault weapon. And the shooter(s) used it at a US Military office to kill people there.
At this point, it's unclear if the shooter(s) was/were military staff or civilian(s). There are now even reports of potential multiple shooters. The Navy Yard ad surrounding neighborhood are still swamped with first responders, as the Navy Yard is still being treated as an active emergency.
So another day, another tragedy. Another week, another horrific incident of gun violence. And we still must wonder what it will take for us to finally stop this bloody madness.