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In this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, Aaron Alexis enters Building #197 at 8:08 a.m., carrying a backpack September 16, 2013 in Washington, DC. Alexis had legitimate access to the Washington Navy Yard as a result of his work as a contractor, and he utilized a valid pass to gain entry to the building. The FBI investigation indicates that the gunman, Aaron Alexis, acted under the delusion that he was being controlled by extremely low frequency (ELF) electromagnetic waves when he carried out his attack on the Navy Yard, in which 12 people were killed and 4 were wounded. (Photo by FBI via Getty Images) ( 181799709 )

In this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, Aaron Alexis drives his rental car, a blue Toyota Prius, through the Washington Navy Yard main gate September 16, 2013 in Washington, DC. (Photo by FBI via Getty Images) ( 181799712 )

In this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, Aaron Alexis Remington 870 shotgun with sawed-off barrel and stock are displayed September 16, 2013 in Washington, DC. Etching on the right side of the shotgun receiver read "Not what yall say!" (Photo by FBI via Getty Images) ( 181799853 )

This image from video provided by the FBI, shows Aaron Alexis moves through the hallways of Building #197 at the Washington Navy Yard on Monday, Sept. 16, 2013, in Washington, carrying a Remington 870 shotgun. (AP Photo/FBI) ( Navy Yard Shooting )

In this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, Aaron Alexis rental car enters Washington Navy Yard Parking Garage #28 at 7:53 a.m. on September 16, 2013 September 16, 2013 in Washington, DC. (Photo by FBI via Getty Images) ( 181799713 )

n this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, the etching reading, "Not what y'all say!" is visible on the right side of Aaron Alexis shotgun receiver September 16, 2013 in Washington, DC. (Photo by FBI via Getty Images) ( 181799778 )

In this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, the etchings on the left side of Alexis shotgun receiver read "Better off this way!" and "My ELF weapon!" September 16, 2013 in Washington, DC. (Photo by FBI via Getty Images) ( 181799849 )

In this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, etching on the barrel of the Remington 870 shotgun used by Alexis reads "End to the torment!" September 16, 2013 in Washington, DC. (Photo by FBI via Getty Images) ( 181799850 )

This image from video provided by the FBI, shows Aaron Alexis moves through the hallways of Building #197 at the Washington Navy Yard on Monday, Sept. 16, 2013, in Washington, carrying a Remington 870 shotgun. (AP Photo/FBI) ( Navy Yard Shooting )

In this handout frame grab from the Washington Navy Yard provided by the FBI on September 25, 2013, Aaron Alexis backpack is found in the fourth-floor mens bathroom, hanging on the back of a stall door September 16, 2013 in Washington, DC. (Photo by FBI via Getty Images) ( 181799854 )