The FBI has just released video of the shooting at the Washington Navy Yard. Clearly Aaron Alexis is seen moving around an office building with a shotgun, looking inside doorways for someone to shoot.
He'd also etched into the shotgun the phrases “End to the torment,” “Not what yall say” and “Better off this way,” as well as "My ELF weapon" -- likely a reference to extremely low frequency electromagnetic waves.
Alexis was under the impression he was under assault from the waves. Aaron Alexis bought the weapon legally in Virginia two days before this rampage.
Screen shot from You Tube video linked