Releasing a series of images which show Aaron Alexis making his way through the building, authorities said he did not target specific individuals.
Records recovered from Alexis’s computer and mobile phone also reveal paranoia and mental health problems.
“Ultra-low frequency attack is what I’ve been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this,” read a document agents recovered from Alexis after the shooting. He had also written “my ELF weapon” – an apparent reference to extremely low frequency waves – on a shotgun he used in the rampage.
Alexis, a former Navy reservist and IT contractor, used a valid badge to access the base on 16 September with a sawn-off Remington shotgun he had bought two days earlier. He was killed by a US park policeman following a shootout that police now say lasted for about an hour.
Authorities say Alexis had only recently started his job. Although there was a “routine performance-related issue addressed to him” on the Friday before the shooting, there is no indication he targeted particular co-workers or was motivated by problems in the workplace, said Valerie Parlave, head of the FBI’s Washington field office.
Ms Parlave said: “At this point I can confirm that there are multiple indicators that Alexis held a delusional belief that he was being controlled or influenced by extremely low frequency, or ELF, electromagnetic waves.
“The etching… is believed to reference these electromagnetic waves. In addition, a document retrieved from the electronic media stated ‘ELF attack is what I’ve been subject to for the last three months, and to be perfectly honest that is what has driven me to this’.”
Ms Parlave added: “ELF technology was a legitimate programme for naval subtonal submarine communications.
“However, conspiracy theories exist which misinterpret its application as the weaponisation of remote neural frequencies for government monitoring and manipulation of citizens.”
Deputy secretary Ash Carter said the department will complete three separate reviews in late December, including internal and independent assessments of base security and clearance processes.
“Bottom line is, we need to know how an employee was able to bring a weapon and ammunition onto a defence installation, and how warning flags were either missed, ignored or not addressed in a timely manner,” Mr Carter said.
He added that the reviews will include consideration of Navy Secretary Ray Mabus’ recommendation that the department require all police reports, not just arrests or convictions, involving an individual be included in background checks.