The FBI has said Washington Navy Yard shooter Aaron Alexis believed he was being controlled by extremely low frequency radio waves.
FBI assistant director Valerie Parlave told a news conference that its evidence showed Alexis acted alone in the rampage that killed 12 people.
"We have found relevant communications on his electronic media which referenced the delusional belief that he was being controlled or influenced by extremely low-frequency electromagnetic waves for the past three months," Ms Parlave said.
Alexis, a government technology contractor, acted alone and was killed by police on the third floor of the Yard's Naval SeaSystems Command headquarters, after exchanging fire with officers for an hour, she said.
The former Navy reservist was working as an IT subcontractor for Hewlett Packard.
The FBI has also released CCTV footage and images of Alexis as he carried out the attack.
