WASHINGTON (AP) -- Washington Navy Yard gunman Aaron Alexis left a note saying he was driven to kill by months of bombardment with extremely low-frequency radio waves, the FBI said Wednesday in a disclosure that explains the phrase he etched on his shotgun: "My ELF Weapon!"
Alexis did not target particular individuals during the Sept. 16 attack in which he killed 12 people, and there is no indication the shooting stemmed from any workplace dispute, said Valerie Parlave, head of the FBI's Washington field office.
Instead, authorities said, his behavior in the weeks before the shooting and records later recovered from the hotel room where he was staying reveal a man increasingly in the throes of paranoia and delusions.
"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 an electronic document agents recovered after the shooting.
The attack came one month after Alexis had complained to police in Rhode Island that people were talking to him through the walls and ceilings of his hotel room and sending microwave vibrations into his body to deprive him of sleep.
He scrawled "My ELF Weapon!" -- an apparent reference to extremely low-frequency waves -- on the shotgun, along with "End to the Torment!" "Not what yall say" and "Better off this way."
Alexis, a 34-year-old former Navy reservist and computer technician for a defense contractor, used a valid badge to get into the Navy Yard with a sawed-off Remington shotgun he had legally purchased two days earlier.
He was killed by a U.S. Park Police officer on the building's third floor following a rampage the FBI said lasted about an hour.
The FBI said it believes he was prepared to die when he went on the murderous attack.
Surveillance video released by the FBI on Wednesday shows Alexis pulling his rental car into a garage, walking into the building with a bag and then pacing deliberately down a corridor with a shotgun, ducking and crouching around a corner and walking briskly down a flight of stairs.
Alexis had started his job as a contractor in the building just a week before.
Although there was a "routine performance-related issue addressed to him" on the Friday before the Monday morning shooting, "there is no indication that this caused any sort of reaction from him," Parlave said.
"We have not determined there to be any previous relationship between Alexis and any of the victims," she said. "There is no evidence or information at this point that indicates he targeted anyone he worked for or worked with. We do not see any one event as triggering this attack."
The shooting revealed flaws in the issuing of security clearances.
Alexis maintained his clearance despite past brushes with the law. Defense officials said he also lied about an arrest and failed to disclose thousands of dollars in debts.
At the Pentagon on Wednesday, Deputy Secretary Ash Carter said the department will complete three reviews in late December, including assessments of base safety procedures and the security clearance process.
"Bottom line is, we need to know how an employee was able to bring a weapon and ammunition onto a DoD installation, and how warning flags were either missed, ignored or not addressed in a timely manner," Carter said.
Carter said the reviews will include consideration of Navy Secretary Ray Mabus' recommendation that the department require that all police reports -- not just arrests or convictions -- be included in background checks.