The FBI says the same gun was used to shoot at the Pentagon and the national Marine Corps Museum in Northern Virginia.
In addition, a Marine Corps recruiting station in Chantilly, Va., outside Washington, was shot at overnight Monday. Investigators are now conducting ballistics tests to determine if the recruiting station shooting is related to the previous incidents.
The FBI said Tuesday it has launched an investigation into all three shootings.
Early on Oct. 19, shots were fired overnight into two windows at the sprawling Defense Department across the Potomac River in suburban Virginia. No one was injured. Two days earlier, bullet holes were discovered in the windows at the National Museum of the Marine Corps in Triangle, Va., 30 miles south of the Pentagon.