Assistant FBI Director Valerie Parlave said during a news conference that a multiagency investigation had determined Aaron Alexis, 34, killed 12 people and wounded three others at random as he moved through the naval complex building before law enforcement officers killed him Sept. 16.
"There are multiple indicators that Alexis held a delusional belief that he was being controlled or influenced by low-frequency electromagnetic waves,'' Parlave said.
She said investigators found documentation in which Alexis stated, "Ultra-low frequency 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.''
Alexis had scrawled "My ELF weapon" on the shotgun he used in the attack. Parlave said "ELF'' was an apparent reference to extremely low-frequency waves, a technology used for submarine communications, but which conspiracy theorists contend is used for government monitoring and manipulation of people.
Parlave said investigators concluded none of the victims had been hit by friendly fire and Alexis had not told anyone of his intentions to go on a rampage with a shotgun he had bought just two days prior.
She said while Alexis, a private contractor in an information technology project at the complex, had "routine performance-related issue" at work the Friday before, there was no indication it had upset him, and there was no relationship between him and any of his victims.
"There is no indication he target anyone he worked for or worked with," she said.
White House press secretary Jay Carney said Wednesday President Barack Obama will keep exhorting Congress "to do the right thing" and pass legislation aimed at reducing gun violence while at the same time the administration will take steps that don't require legislative action.
"I think our record on pursuing common-sense measures to reduce gun violence this year is robust, to say the least," Carney said. "And the president will continue to push that agenda, as I think he made clear."
Three of the victims of the mass shooting were remembered Tuesday at memorial services.
Services for John Johnson, 73, in Gaithersburg, Md., and Mary Frances DeLorenzo Knight, 51, in Virginia's Fairfax County were private, The Washington Post reported.
Hundreds of people gathered at a funeral home in Woodbridge, Va., for a service for Kathleen Gaarde, 62, a financial analyst who had worked for the Navy for more than 30 years.
Her husband, Douglass, said in his eulogy that Kathleen would have disliked the pomp of the service, the sailors standing at attention as a trumpeter played taps and the flag handed him by an admiral.
"Tough," he told his wife, looking upward.
Gaarde talked of how he and his wife had been apart for less than two months in their 43-year marriage. He came close to tears as he described their plans for retirement.