(WASHINGTON) -- Aaron Alexis, the man who carried out the massacre in the Washington Navy Yard last week, had a "delusional belief" that he was being controlled by electromagnetic waves and fully expected to be killed when he launched his attack, federal investigators said Wednesday.
Alexis etched the phrase "end to the torment" on the shotgun's barrel, officials said.
Authorities also found a note from Alexis in which he complained that he had been under "ultra low frequency attack" for three months.
"To be perfectly honest, that is what has driven me to this," he wrote in his farewell message, federal investigators disclosed Wednesday.
That note is one of "multiple indicators" found on Alexis' electronic devices seized after the attack showing that he "held a delusional belief that he was being controlled or influenced" by electromagnetic waves, the head of the FBI's Washington Field Office told reporters at a press conference.
He was "prepared to die during the attack," and he "accepted death as the inevitable consequence of his actions," Valerie Parlave said.
The shotgun he brought with him into the Navy Yard's Building 197 had been altered, with a sawed-off barrel and stock. Only days before the attack, he bought a shotgun at a Virginia gun store and a hacksaw at a local Home Depot.
Parlave confirmed previously reported details, including that the phrases "better off this way" and "my ELF weapon" were etched into the shotgun. Authorities believed "ELF" refers to "extremely low frequency," a form of Naval communications that conspiracy theorists believe is used by the U.S. government to monitor its citizens, Parlave said.
In addition to those phrases, Alexis etched the phrase "end to the torment" on the shotgun's barrel and the phrase "not what y'all say" on the weapon's receiver.
After the press conference, authorities released video and images of Alexis entering Building 197 at about 8:08 a.m., and then walking in a hallway and stairwell carrying the shotgun.
"Over the course of approximately one hour, Alexis shot and killed 12 victims and wounded four surviving victims," before being killed himself, Parlave said.
Investigators have concluded Alexis was not specifically targeting anyone during his rampage.
Parlave described Building 197 as "a tactical nightmare" for law enforcement trying to track him down, with lots of places for him to hide.
As for Alexis' troubled mental health history, investigators said they are still looking into that. They refused to offer any details about a series of troubling and bizarre encounters with law enforcement and strangers in the weeks before the Navy Yard massacre.
In the early days of August, Alexis check into military housing in Newport, R.I., but he abruptly left after complaining of hearing noise coming from the linen closet. Around the same time, Alexis called Newport, R.I., police to complain about hearing voices in his head and being under surveillance by shadowy forces.
Rhode Island police alerted Newport Naval Station authorities about Alexis' bizarre story, but the information was never passed up the chain of command, the Navy has confirmed. Alexis kept his job as a civilian contractor with the Navy, holding a secret security clearance.
He launched his deadly assault within a week of first reporting for duty as a "server refresher" at the Navy Yard.
Video surveillance from that morning shows he first went up to the fourth floor and entered a men's restroom there, carrying a backpack, according to court documents.
After creating initial carnage on the fourth floor, Alexis returned to the first floor, where he fired on a security guard and took his Beretta handgun, FBI officials have said. Alexis then worked his way back to the third and fourth floors, "moving without particular purpose" and gunning down even more victims in the hallway and in their offices, FBI Director James Comey recently told reporters.
Court documents say Naval Criminal Investigative personnel were the first to respond to the attack and engaged Alexis. Members of the D.C. Metropolitan Police Department's Emergency Response Team later arrived and also exchanged gunfire with Alexis, according to court documents. His body was later found on the third floor.
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