The FBI said there is no indication that Alexis targeted any specific individuals or knew any of his victims when he opened fire inside a building on Sept. 16, killing 12 people.
Valerie Parlave, head of the FBI's field office in Washington, said at a news conference Wednesday that investigators are continuing to explore the background and motivations of the 34-year-old former navy reservist and contractor. However, she said Alexis had a well-documented history of mental health issues.
Parlave said Alexis held a delusional belief that he was being controlled by extremely low frequency waves, or ELF waves.
Alexis had written "my ELF weapon" and “end to the torment” on a shotgun he used in last week's rampage.
An electronic document recovered after the shooting read: "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."
The FBI believes Alexis was prepared to die when he went on the murderous attack.
His rampage came one month after he had complained to police in Rhode Island that people were talking to him through the walls and ceilings of a hotel room and sending microwave vibrations into his body to deprive him of sleep, the FBI said.
The FBI said Alexis purchased a Remington 870 shotgun and ammunition at a gun shop in northern Virginia two days before returning to the navy yard. He also purchased a hacksaw at a home improvement store in Northern Virginia, the FBI said in a statement posted on its website.
Alexis had legitimate access to the navy yard as a result of his work as a contractor and he used a valid ID pass to gain entry to Building 197, where he used the shotgun, as well as a handgun which he obtained in the building, to kill 12 people and wound four others before he was shot and killed by police.
The FBI believes Alexis acted alone.