Access to the Navy Yard is being limited to mission-essential personnel.
In addition to the 12 who died, eight others were wounded. All of them are expected to survive.
Authorities continue to learn more about the gunman -- a defense contract employee and former Navy reservist who used a valid pass to get onto the installation and started firing inside a building. He was eventually killed in a gun battle with police.
According to two federal law enforcement officials, Aaron Alexis carried three weapons: an AR-15 assault rifle, a shotgun, and a handgun he took from a police officer at the scene. The AR-15 is the same type of rifle that was used in last year's mass shooting at a Connecticut elementary school. It was also used in the shooting at a Colorado movie theater that killed 12 people.