Source [The FBI] .. http://www.fbi.gov/news/navy-yard-shootings-investigation
Navy Yard Shootings
On the morning of Monday, September 16, 2013, shooter Aaron Alexis arrived at the Washington Navy Yard in Southeast, Washington, D.C.
Alexis had legitimate access to the Navy Yard as a result of his work as a contractor and he utilized a valid pass to gain entry to Building #197.
Shortly after his arrival in the building and over the course of approximately one hour, Alexis used the Remington 870 shotgun and a Beretta handgun he obtained during the course of his shooting to kill 12 victims and wound four surviving victims before he was shot and killed by law enforcement officers.
Investigation to date has determined that Alexis acted alone.