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.
An official says Alexis is believed to have gotten into the Navy Yard by using someone else's identification card. It is not yet clear if that individual was an accomplice or if that person's ID card was stolen.
Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget.
It builds, buys and maintains the Navy's ships and submarines and their combat systems.
Police from multiple local and federal agencies were responding along with SWAT crews.
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