"We have found relevant communications on his electronic media which referenced the delusional belief that he was being controlled or influenced by extremely low frequency electromagnetic waves for the past three months," Parlave told a news conference.
Surveillance video released by the FBI showed Alexis driving into a Navy Yard parking garage in a rented blue Toyota Prius shortly before 8 a.m. Carrying a backpack, he then entered the Naval Sea Systems Command building, site of the shootings, through a door.
The brief video also shows Alexis, armed with a Remington shotgun and dressed in dark clothing, walking down a stairway and corridors in a crouch with the weapon held at the ready.
People can be glimpsed at the end of one corridor. At one point, he aims the shotgun into a room but does not fire.
Parlave said Alexis, a government technology contractor, had been armed with the shotgun, which had a sawed-off barrel and stock, and a pistol he obtained during the shooting.
"MY ELF WEAPON"