WASHINGTON – Washington Navy Yard killer Aaron Alexis left a note saying he was driven to kill by bombardment with extremely low-frequency radio waves, the FBI said Wednesday, 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, assistant director of the FBI’s Washington field office.

Instead, authorities said, his behavior in the weeks before the shooting and records later recovered from his hotel room reveal a man increasingly in the throes of paranoia and delusions.

“Ultralow 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 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.

The message scrawled on his shotgun was an apparent reference to extremely low frequency waves, along with “End to the Torment!” “Not what yall say” and “Better off this way,” the FBI said.

Alexis, a 34-year-old former U.S. Navy reservist and computer technician for a government contractor, used a valid badge to get into the Washington Navy Yard and opened fire inside a building with the sawed-off Remington shotgun, which he had legally purchased in Virginia two days earlier. He also used a 9mm handgun that he took from a security guard.

He was killed in the building by a U.S. Park Police officer following a rampage the FBI said lasted more than an hour.

“There are indicators that Alexis was prepared to die during the attack and that he accepted death as the inevitable consequence of his actions,” Parlave said.

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 skulking down a corridor with a shotgun, ducking and crouching around a corner and walking briskly down a flight of stairs.

A timeline issued by the FBI shows Alexis started the rampage on the fourth floor and then moved down to the third and first floors. He ultimately returned to the third floor, where he was killed around 9:25 a.m.

Alexis had started a 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.